We Keep Coding

Overwrite row if exists [duplicate]

Several months ago I learned from an answer on Stack Overflow how to perform multiple updates at once in MySQL using the following syntax:
INSERT INTO table (id, field, field2) VALUES (1, A, X), (2, B, Y), (3, C, Z)
ON DUPLICATE KEY UPDATE field=VALUES(Col1), field2=VALUES(Col2);
I've now switched over to PostgreSQL and apparently this is not correct. It's referring to all the correct tables so I assume it's a matter of different keywords being used but I'm not sure where in the PostgreSQL documentation this is covered.
To clarify, I want to insert several things and if they already exist to update them.

PostgreSQL since version 9.5 has UPSERT syntax, with ON CONFLICT clause. with the following syntax (similar to MySQL)
INSERT INTO the_table (id, column_1, column_2)
VALUES (1, 'A', 'X'), (2, 'B', 'Y'), (3, 'C', 'Z')
ON CONFLICT (id) DO UPDATE
SET column_1 = excluded.column_1,
column_2 = excluded.column_2;
Searching postgresql's email group archives for "upsert" leads to finding an example of doing what you possibly want to do, in the manual:
Example 38-2. Exceptions with UPDATE/INSERT
This example uses exception handling to perform either UPDATE or INSERT, as appropriate:
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
-- note that "a" must be unique
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing, and loop to try the UPDATE again
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
There's possibly an example of how to do this in bulk, using CTEs in 9.1 and above, in the hackers mailing list:
WITH foos AS (SELECT (UNNEST(%foo[])).*)
updated as (UPDATE foo SET foo.a = foos.a ... RETURNING foo.id)
INSERT INTO foo SELECT foos.* FROM foos LEFT JOIN updated USING(id)
WHERE updated.id IS NULL;
See a_horse_with_no_name's answer for a clearer example.

Warning: this is not safe if executed from multiple sessions at the same time (see caveats below).
Another clever way to do an "UPSERT" in postgresql is to do two sequential UPDATE/INSERT statements that are each designed to succeed or have no effect.
UPDATE table SET field='C', field2='Z' WHERE id=3;
INSERT INTO table (id, field, field2)
SELECT 3, 'C', 'Z'
WHERE NOT EXISTS (SELECT 1 FROM table WHERE id=3);
The UPDATE will succeed if a row with "id=3" already exists, otherwise it has no effect.
The INSERT will succeed only if row with "id=3" does not already exist.
You can combine these two into a single string and run them both with a single SQL statement execute from your application. Running them together in a single transaction is highly recommended.
This works very well when run in isolation or on a locked table, but is subject to race conditions that mean it might still fail with duplicate key error if a row is inserted concurrently, or might terminate with no row inserted when a row is deleted concurrently. A SERIALIZABLE transaction on PostgreSQL 9.1 or higher will handle it reliably at the cost of a very high serialization failure rate, meaning you'll have to retry a lot. See why is upsert so complicated, which discusses this case in more detail.
This approach is also subject to lost updates in read committed isolation unless the application checks the affected row counts and verifies that either the insert or the update affected a row.

With PostgreSQL 9.1 this can be achieved using a writeable CTE (common table expression):
WITH new_values (id, field1, field2) as (
values
(1, 'A', 'X'),
(2, 'B', 'Y'),
(3, 'C', 'Z')
),
upsert as
(
update mytable m
set field1 = nv.field1,
field2 = nv.field2
FROM new_values nv
WHERE m.id = nv.id
RETURNING m.*
)
INSERT INTO mytable (id, field1, field2)
SELECT id, field1, field2
FROM new_values
WHERE NOT EXISTS (SELECT 1
FROM upsert up
WHERE up.id = new_values.id)
See these blog entries:
Upserting via Writeable CTE
WAITING FOR 9.1 – WRITABLE CTE
WHY IS UPSERT SO COMPLICATED?
Note that this solution does not prevent a unique key violation but it is not vulnerable to lost updates.
See the follow up by Craig Ringer on dba.stackexchange.com

In PostgreSQL 9.5 and newer you can use INSERT ... ON CONFLICT UPDATE.
See the documentation.
A MySQL INSERT ... ON DUPLICATE KEY UPDATE can be directly rephrased to a ON CONFLICT UPDATE. Neither is SQL-standard syntax, they're both database-specific extensions. There are good reasons MERGE wasn't used for this, a new syntax wasn't created just for fun. (MySQL's syntax also has issues that mean it wasn't adopted directly).
e.g. given setup:
CREATE TABLE tablename (a integer primary key, b integer, c integer);
INSERT INTO tablename (a, b, c) values (1, 2, 3);
the MySQL query:
INSERT INTO tablename (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE c=c+1;
becomes:
INSERT INTO tablename (a, b, c) values (1, 2, 10)
ON CONFLICT (a) DO UPDATE SET c = tablename.c + 1;
Differences:
You must specify the column name (or unique constraint name) to use for the uniqueness check. That's the ON CONFLICT (columnname) DO
The keyword SET must be used, as if this was a normal UPDATE statement
It has some nice features too:
You can have a WHERE clause on your UPDATE (letting you effectively turn ON CONFLICT UPDATE into ON CONFLICT IGNORE for certain values)
The proposed-for-insertion values are available as the row-variable EXCLUDED, which has the same structure as the target table. You can get the original values in the table by using the table name. So in this case EXCLUDED.c will be 10 (because that's what we tried to insert) and "table".c will be 3 because that's the current value in the table. You can use either or both in the SET expressions and WHERE clause.
For background on upsert see How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?

I was looking for the same thing when I came here, but the lack of a generic "upsert" function botherd me a bit so I thought you could just pass the update and insert sql as arguments on that function form the manual
that would look like this:
CREATE FUNCTION upsert (sql_update TEXT, sql_insert TEXT)
RETURNS VOID
LANGUAGE plpgsql
AS $$
BEGIN
LOOP
-- first try to update
EXECUTE sql_update;
-- check if the row is found
IF FOUND THEN
RETURN;
END IF;
-- not found so insert the row
BEGIN
EXECUTE sql_insert;
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing and loop
END;
END LOOP;
END;
$$;
and perhaps to do what you initially wanted to do, batch "upsert", you could use Tcl to split the sql_update and loop the individual updates, the preformance hit will be very small see http://archives.postgresql.org/pgsql-performance/2006-04/msg00557.php
the highest cost is executing the query from your code, on the database side the execution cost is much smaller

There is no simple command to do it.
The most correct approach is to use function, like the one from docs.
Another solution (although not that safe) is to do update with returning, check which rows were updates, and insert the rest of them
Something along the lines of:
update table
set column = x.column
from (values (1,'aa'),(2,'bb'),(3,'cc')) as x (id, column)
where table.id = x.id
returning id;
assuming id:2 was returned:
insert into table (id, column) values (1, 'aa'), (3, 'cc');
Of course it will bail out sooner or later (in concurrent environment), as there is clear race condition in here, but usually it will work.
Here's a longer and more comprehensive article on the topic.

I use this function merge
CREATE OR REPLACE FUNCTION merge_tabla(key INT, data TEXT)
RETURNS void AS
$BODY$
BEGIN
IF EXISTS(SELECT a FROM tabla WHERE a = key)
THEN
UPDATE tabla SET b = data WHERE a = key;
RETURN;
ELSE
INSERT INTO tabla(a,b) VALUES (key, data);
RETURN;
END IF;
END;
$BODY$
LANGUAGE plpgsql

Personally, I've set up a "rule" attached to the insert statement. Say you had a "dns" table that recorded dns hits per customer on a per-time basis:
CREATE TABLE dns (
"time" timestamp without time zone NOT NULL,
customer_id integer NOT NULL,
hits integer
);
You wanted to be able to re-insert rows with updated values, or create them if they didn't exist already. Keyed on the customer_id and the time. Something like this:
CREATE RULE replace_dns AS
ON INSERT TO dns
WHERE (EXISTS (SELECT 1 FROM dns WHERE ((dns."time" = new."time")
AND (dns.customer_id = new.customer_id))))
DO INSTEAD UPDATE dns
SET hits = new.hits
WHERE ((dns."time" = new."time") AND (dns.customer_id = new.customer_id));
Update: This has the potential to fail if simultaneous inserts are happening, as it will generate unique_violation exceptions. However, the non-terminated transaction will continue and succeed, and you just need to repeat the terminated transaction.
However, if there are tons of inserts happening all the time, you will want to put a table lock around the insert statements: SHARE ROW EXCLUSIVE locking will prevent any operations that could insert, delete or update rows in your target table. However, updates that do not update the unique key are safe, so if you no operation will do this, use advisory locks instead.
Also, the COPY command does not use RULES, so if you're inserting with COPY, you'll need to use triggers instead.

Similar to most-liked answer, but works slightly faster:
WITH upsert AS (UPDATE spider_count SET tally=1 WHERE date='today' RETURNING *)
INSERT INTO spider_count (spider, tally) SELECT 'Googlebot', 1 WHERE NOT EXISTS (SELECT * FROM upsert)
(source: http://www.the-art-of-web.com/sql/upsert/)

I custom "upsert" function above, if you want to INSERT AND REPLACE :
`
CREATE OR REPLACE FUNCTION upsert(sql_insert text, sql_update text)
RETURNS void AS
$BODY$
BEGIN
-- first try to insert and after to update. Note : insert has pk and update not...
EXECUTE sql_insert;
RETURN;
EXCEPTION WHEN unique_violation THEN
EXECUTE sql_update;
IF FOUND THEN
RETURN;
END IF;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION upsert(text, text)
OWNER TO postgres;`
And after to execute, do something like this :
SELECT upsert($$INSERT INTO ...$$,$$UPDATE... $$)
Is important to put double dollar-comma to avoid compiler errors
check the speed...

According the PostgreSQL documentation of the INSERT statement, handling the ON DUPLICATE KEY case is not supported. That part of the syntax is a proprietary MySQL extension.

I have the same issue for managing account settings as name value pairs.
The design criteria is that different clients could have different settings sets.
My solution, similar to JWP is to bulk erase and replace, generating the merge record within your application.
This is pretty bulletproof, platform independent and since there are never more than about 20 settings per client, this is only 3 fairly low load db calls - probably the fastest method.
The alternative of updating individual rows - checking for exceptions then inserting - or some combination of is hideous code, slow and often breaks because (as mentioned above) non standard SQL exception handling changing from db to db - or even release to release.
#This is pseudo-code - within the application:
BEGIN TRANSACTION - get transaction lock
SELECT all current name value pairs where id = $id into a hash record
create a merge record from the current and update record
(set intersection where shared keys in new win, and empty values in new are deleted).
DELETE all name value pairs where id = $id
COPY/INSERT merged records
END TRANSACTION

CREATE OR REPLACE FUNCTION save_user(_id integer, _name character varying)
RETURNS boolean AS
$BODY$
BEGIN
UPDATE users SET name = _name WHERE id = _id;
IF FOUND THEN
RETURN true;
END IF;
BEGIN
INSERT INTO users (id, name) VALUES (_id, _name);
EXCEPTION WHEN OTHERS THEN
UPDATE users SET name = _name WHERE id = _id;
END;
RETURN TRUE;
END;
$BODY$
LANGUAGE plpgsql VOLATILE STRICT

For merging small sets, using the above function is fine. However, if you are merging large amounts of data, I'd suggest looking into http://mbk.projects.postgresql.org
The current best practice that I'm aware of is:
COPY new/updated data into temp table (sure, or you can do INSERT if the cost is ok)
Acquire Lock [optional] (advisory is preferable to table locks, IMO)
Merge. (the fun part)

UPDATE will return the number of modified rows. If you use JDBC (Java), you can then check this value against 0 and, if no rows have been affected, fire INSERT instead. If you use some other programming language, maybe the number of the modified rows still can be obtained, check documentation.
This may not be as elegant but you have much simpler SQL that is more trivial to use from the calling code. Differently, if you write the ten line script in PL/PSQL, you probably should have a unit test of one or another kind just for it alone.

Edit: This does not work as expected. Unlike the accepted answer, this produces unique key violations when two processes repeatedly call upsert_foo concurrently.
Eureka! I figured out a way to do it in one query: use UPDATE ... RETURNING to test if any rows were affected:
CREATE TABLE foo (k INT PRIMARY KEY, v TEXT);
CREATE FUNCTION update_foo(k INT, v TEXT)
RETURNS SETOF INT AS $$
UPDATE foo SET v = $2 WHERE k = $1 RETURNING $1
$$ LANGUAGE sql;
CREATE FUNCTION upsert_foo(k INT, v TEXT)
RETURNS VOID AS $$
INSERT INTO foo
SELECT $1, $2
WHERE NOT EXISTS (SELECT update_foo($1, $2))
$$ LANGUAGE sql;
The UPDATE has to be done in a separate procedure because, unfortunately, this is a syntax error:
... WHERE NOT EXISTS (UPDATE ...)
Now it works as desired:
SELECT upsert_foo(1, 'hi');
SELECT upsert_foo(1, 'bye');
SELECT upsert_foo(3, 'hi');
SELECT upsert_foo(3, 'bye');

PostgreSQL >= v15
Big news on this topic as in PostgreSQL v15, it is possible to use MERGE command. In fact, this long awaited feature was listed the first of the improvements of the v15 release.
This is similar to INSERT ... ON CONFLICT but more batch-oriented. It has a powerful WHEN MATCHED vs WHEN NOT MATCHED structure that gives the ability to INSERT, UPDATE or DELETE on such conditions.
It not only eases bulk changes, but it even adds more control that tradition UPSERT and INSERT ... ON CONFLICT
Take a look at this very complete sample from official page:
MERGE INTO wines w
USING wine_stock_changes s
ON s.winename = w.winename
WHEN NOT MATCHED AND s.stock_delta > 0 THEN
INSERT VALUES(s.winename, s.stock_delta)
WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN
UPDATE SET stock = w.stock + s.stock_delta
WHEN MATCHED THEN
DELETE;
PostgreSQL v9, v10, v11, v12, v13, v14
If version is under v15 and over v9.5 , probably best choice is to use UPSERT syntax, with ON CONFLICT clause

Here is the example how to do upsert with params and without special sql constructions
if you have special condition (sometimes you can't use 'on conflict' because you can't create constraint)
WITH upd AS
(
update view_layer set metadata=:metadata where layer_id = :layer_id and view_id = :view_id returning id
)
insert into view_layer (layer_id, view_id, metadata)
(select :layer_id layer_id, :view_id view_id, :metadata metadata FROM view_layer l
where NOT EXISTS(select id FROM upd WHERE id IS NOT NULL) limit 1)
returning id
maybe it will be helpful

pyodbc join tables with equal named columns ("upsert") [duplicate]

I need to write an SQL query for MS-Access 2000 so that a row is updated if it exists, but inserted if it does not. (I believe this is called an "upsert")
i.e.
If row exists...
UPDATE Table1 SET (...) WHERE Column1='SomeValue'
If it does not exist...
INSERT INTO Table1 VALUES (...)
Can this be done in one query?

You can simulate an upsert in an Access by using an UPDATE query with a LEFT JOIN.
update b
left join a on b.id=a.id
set a.f1=b.f1
, a.f2=b.f2
, a.f3=b.f3

Assuming a unique index on Column1, you can use a DCount expression to determine whether you have zero or one row with Column1 = 'SomeValue'. Then INSERT or UPDATE based on that count.
If DCount("*", "Table1", "Column1 = 'SomeValue'") = 0 Then
Debug.Print "do INSERT"
Else
Debug.Print "do UPDATE"
End If
I prefer this approach to first attempting an INSERT, trapping the 3022 key violation error, and doing an UPDATE in response to the error. However I can't claim huge benefits from my approach. If your table includes an autonumber field, avoiding a failed INSERT would stop you from expending the next autonumber value needlessly. I can also avoid building an INSERT string when it's not needed. The Access Cookbook told me string concatenation is a moderately expensive operation in VBA, so I look for opportunities to avoid building strings unless they're actually needed. This approach will also avoid creating a lock for an unneeded INSERT.
However, none of those reasons may be very compelling for you. And in all honesty I think my preference in this case may be about what "feels right" to me. I agree with this comment by #David-W-Fenton to a previous Stack Overflow question: "It's better to write your SQL so you don't attempt to append values that already exist -- i.e., prevent the error from happening in the first place rather than depending on the database engine to save you from yourself."

An "upsert" is possible, if the tables have a unique key.
This old tip from Smart Access is one of my favourites:
Update and Append Records with One Query
By Alan Biggs
Did you know that you can use an update query in Access to both update
and add records at the same time? This is useful if you have two
versions of a table, tblOld and tblNew, and you want to integrate the
changes from tblNew into tblOld.
Follow these steps:
Create an update query and add the two tables. Join the two tables by
dragging the key field of tblNew onto the matching field of tblOld.
Double-click on the relationship and choose the join option that includes all records from tblNew and only those that match from
tblOld.
Select all the fields from tblOld and drag them onto the QBE grid.
For each field, in the Update To cell type in tblNew.FieldName, where FieldName matches the field name of tblOld.
Select Query Properties from the View menu and change Unique Records to False. (This switches off the DISTINCTROW option in the SQL
view. If you leave this on you'll get only one blank record in your
results, but you want one blank record for each new record to be added
to tblOld.)
Run the query and you'll see the changes to tblNew are now in tblOld.
This will only add records to tblOld that have been added to tblNew.
Records in tblOld that aren't present in tblNew will still remain in
tblOld.

I usually run the insert statement first and then I check to see if error 3022 occurred, which indicates the row already exists. So something like this:
On Error Resume Next
CurrentDb.Execute "INSERT INTO Table1 (Fields) VALUES (Data)", dbFailOnError
If Err.Number = 3022 Then
Err.Clear
CurrentDb.Execute "UPDATE Table1 SET (Fields = Values) WHERE Column1 = 'SomeValue'", dbFailOnError
ElseIf Err.Number <> 0 Then
'Handle the error here
Err.Clear
End If
Edit1:
I want to mention that what I've posted here is a very common solution but you should be aware that planning on errors and using them as part of the normal flow of your program is generally considered a bad idea, especially if there are other ways of achieving the same results. Thanks to RolandTumble for pointing this out.

You don't need to catch the error. Instead, just run the INSERT statement and then check
CurrentDb.RecordsAffected
It will either be 1 or 0, depending.
Note: It's not good practice to execute against CurrentDB. Better to capture the database to a local variable:
Dim db As DAO.Database
Set db = CurrentDb
db.Execute(INSERT...)
If db.RecordsAffected = 0 Then
db.Execute(UPDATE...)
End If

As others have mentioned, You can UPSERT with an UPDATE LEFT JOIN using the new table as the left hand side. This will add all missing records and update matching records, leaving deleted records intact.
If we follow the Create and run an update query Article we will end up with SQL that looks like this:
UPDATE Table1
INNER JOIN NewTable1 ON Table1.ID = NewTable1.ID
SET Table1.FirstName = [NewTable1].[FirstName]
but an inner join will only update matching records, it won't add new records. So let's change that INNER to a LEFT:
UPDATE Table1
LEFT JOIN NewTable1 ON Table1.ID = NewTable1.ID
SET Table1.FirstName = [NewTable1].[FirstName]
Now save a copy of the DB. Run a test on the copy before you run this on your primary DB.

Mysql 'VALUES function' is deprecated

This is my python code which prints the sql query.
def generate_insert_statement(column_names, values_format, table_name, items, insert_template=INSERT_TEMPLATE, ):
return insert_template.format(
column_names=",".join(column_names),
values=",".join(
map(
lambda x: generate_raw_values(values_format, x),
items
)
),
table_name=table_name,
updates_on=create_updates_on_columns(column_names)
)
query = generate_insert_statement(table_name=property['table_name'],
column_names=property['column_names'],
values_format=property['values_format'], items=batch)
print(query) #here
execute_commit(query)
When printing the Mysql query my Django project shows following error in the terminal:
'VALUES function' is deprecated and will be removed in a future release. Please use an alias (INSERT INTO ... VALUES (...) AS alias) and replace VALUES(col) in the ON DUPLICATE KEY UPDATE clause with alias.col instead
Mysql doumentation does not say much about it.What does this mean and how to can i rectify it.
INSERT_TEMPLATE = "INSERT INTO {table_name} ({column_names}) VALUES {values} ON DUPLICATE KEY UPDATE {updates_on};"

Basically, mysql is looking toward removing a longstanding non-standard use of the values function to clear the way for some future work where the SQL standard allows using a VALUES keyword for something very different, and because how the VALUES function works in subqueries or not in a ON DUPLICATE KEY UPDATE clause can be surprising.
You need to add an alias to the VALUES clause and then use that alias instead of the non-standard VALUES function in the ON DUPLICATE KEY UPDATE clause, e.g. change
INSERT INTO foo (bar, baz) VALUES (1,2)
ON DUPLICATE KEY UPDATE baz=VALUES(baz)
to
INSERT INTO foo (bar, baz) VALUES (1,2) AS new_foo
ON DUPLICATE KEY UPDATE baz=new_foo.baz
(This only works on mysql 8+, not on older versions or in any version of mariadb through at least 10.8.3)
Note that this is no different if you are updating multiple rows:
INSERT INTO foo (bar, baz) VALUES (1,2),(3,4),(5,6) AS new_foo
ON DUPLICATE KEY UPDATE baz=new_foo.baz
From https://dev.mysql.com/worklog/task/?id=13325:
According to the SQL standard, VALUES is a table value constructor that returns a table. In MySQL this is true for simple INSERT and REPLACE statements, but MySQL also uses VALUES to refer to values in INSERT ... ON DUPLICATE KEY UPDATE statements. E.g.:
INSERT INTO t(a,b) VALUES (1, 2) ON DUPLICATE KEY
UPDATE a = VALUES (b) + 1;
VALUES (b) refers to the value for b in the table value constructor for the INSERT, in this case 2.
To make the value available in simple arithmetic expressions, it is part of the parser rule for simple_expr. Unfortunately, this also means that VALUES can be used in this way in a lot of other statements, e.g.:
SELECT a FROM t WHERE a=VALUES(a);
In all such statements, VALUES returns NULL, so the above query would not have the intended effect. The only meaningful usage of VALUES as a function, rather than a table value constructor, is in INSERT ... ON DUPLICATE KEY UPDATE. Also, the non-standard use in INSERT ... ON DUPLICATE KEY UPDATE does not extend to subqueries. E.g.:
INSERT INTO t1 VALUES(1,2) ON DUPLICATE KEY
UPDATE a=(SELECT a FROM t2 WHERE b=VALUES(b));
This does not do what the user expects. VALUES(b) will return NULL, even if it is in an INSERT .. ON DUPLICATE KEY UPDATE statement.
The non-standard syntax also makes it harder (impossible?) to implement standard behavior of VALUES as specified in feature F641 "Row and table constructors".

I am delete object of model with pk=1, but new object have pk=2 [duplicate]

I have got a table with auto increment primary key. This table is meant to store millions of records and I don't need to delete anything for now. The problem is, when new rows are getting inserted, because of some error, the auto increment key is leaving some gaps in the auto increment ids.. For example, after 5, the next id is 8, leaving the gap of 6 and 7. Result of this is when I count the rows, it results 28000, but the max id is 58000. What can be the reason? I am not deleting anything. And how can I fix this issue.
P.S. I am using insert ignore while inserting records so that it doesn't give error when I try to insert duplicate entry in unique column.

This is by design and will always happen.
Why?
Let's take 2 overlapping transaction that are doing INSERTs
Transaction 1 does an INSERT, gets the value (let's say 42), does more work
Transaction 2 does an INSERT, gets the value 43, does more work
Then
Transaction 1 fails. Rolls back. 42 stays unused
Transaction 2 completes with 43
If consecutive values were guaranteed, every transaction would have to happen one after the other. Not very scalable.
Also see Do Inserted Records Always Receive Contiguous Identity Values (SQL Server but same principle applies)

You can create a trigger to handle the auto increment as:
CREATE DEFINER=`root`#`localhost` TRIGGER `mytable_before_insert` BEFORE INSERT ON `mytable` FOR EACH ROW
BEGIN
SET NEW.id = (SELECT IFNULL(MAX(id), 0) + 1 FROM mytable);;
END

This is a problem in the InnoDB, the storage engine of MySQL.
It really isn't a problem as when you check the docs on “AUTO_INCREMENT Handling in InnoDB” it basically says InnoDB uses a special table to do the auto increments at startup
And the query it uses is something like
SELECT MAX(ai_col) FROM t FOR UPDATE;
This improves concurrency without really having an affect on your data.
To not have this use MyISAM instead of InnoDB as storage engine

Perhaps (I haven't tested this) a solution is to set innodb_autoinc_lock_mode to 0.
According to http://dev.mysql.com/doc/refman/5.7/en/innodb-auto-increment-handling.html this might make things a bit slower (if you perform inserts of multiple rows in a single query) but should remove gaps.

You can try insert like :
insert ignore into table select (select max(id)+1 from table), "value1", "value2" ;
This will try
insert new data with last unused id (not autoincrement)
if in unique fields duplicate entry found ignore it
else insert new data normally
( but this method not support to update fields if duplicate entry found )

Bulk upsert (insert-update) a csv in postgres [duplicate]

A very frequently asked question here is how to do an upsert, which is what MySQL calls INSERT ... ON DUPLICATE UPDATE and the standard supports as part of the MERGE operation.
Given that PostgreSQL doesn't support it directly (before pg 9.5), how do you do this? Consider the following:
CREATE TABLE testtable (
id integer PRIMARY KEY,
somedata text NOT NULL
);
INSERT INTO testtable (id, somedata) VALUES
(1, 'fred'),
(2, 'bob');
Now imagine that you want to "upsert" the tuples (2, 'Joe'), (3, 'Alan'), so the new table contents would be:
(1, 'fred'),
(2, 'Joe'), -- Changed value of existing tuple
(3, 'Alan') -- Added new tuple
That's what people are talking about when discussing an upsert. Crucially, any approach must be safe in the presence of multiple transactions working on the same table - either by using explicit locking, or otherwise defending against the resulting race conditions.
This topic is discussed extensively at Insert, on duplicate update in PostgreSQL?, but that's about alternatives to the MySQL syntax, and it's grown a fair bit of unrelated detail over time. I'm working on definitive answers.
These techniques are also useful for "insert if not exists, otherwise do nothing", i.e. "insert ... on duplicate key ignore".

9.5 and newer:
PostgreSQL 9.5 and newer support INSERT ... ON CONFLICT (key) DO UPDATE (and ON CONFLICT (key) DO NOTHING), i.e. upsert.
Comparison with ON DUPLICATE KEY UPDATE.
Quick explanation.
For usage see the manual - specifically the conflict_action clause in the syntax diagram, and the explanatory text.
Unlike the solutions for 9.4 and older that are given below, this feature works with multiple conflicting rows and it doesn't require exclusive locking or a retry loop.
The commit adding the feature is here and the discussion around its development is here.
If you're on 9.5 and don't need to be backward-compatible you can stop reading now.
9.4 and older:
PostgreSQL doesn't have any built-in UPSERT (or MERGE) facility, and doing it efficiently in the face of concurrent use is very difficult.
This article discusses the problem in useful detail.
In general you must choose between two options:
Individual insert/update operations in a retry loop; or
Locking the table and doing batch merge
Individual row retry loop
Using individual row upserts in a retry loop is the reasonable option if you want many connections concurrently trying to perform inserts.
The PostgreSQL documentation contains a useful procedure that'll let you do this in a loop inside the database. It guards against lost updates and insert races, unlike most naive solutions. It will only work in READ COMMITTED mode and is only safe if it's the only thing you do in the transaction, though. The function won't work correctly if triggers or secondary unique keys cause unique violations.
This strategy is very inefficient. Whenever practical you should queue up work and do a bulk upsert as described below instead.
Many attempted solutions to this problem fail to consider rollbacks, so they result in incomplete updates. Two transactions race with each other; one of them successfully INSERTs; the other gets a duplicate key error and does an UPDATE instead. The UPDATE blocks waiting for the INSERT to rollback or commit. When it rolls back, the UPDATE condition re-check matches zero rows, so even though the UPDATE commits it hasn't actually done the upsert you expected. You have to check the result row counts and re-try where necessary.
Some attempted solutions also fail to consider SELECT races. If you try the obvious and simple:
-- THIS IS WRONG. DO NOT COPY IT. It's an EXAMPLE.
BEGIN;
UPDATE testtable
SET somedata = 'blah'
WHERE id = 2;
-- Remember, this is WRONG. Do NOT COPY IT.
INSERT INTO testtable (id, somedata)
SELECT 2, 'blah'
WHERE NOT EXISTS (SELECT 1 FROM testtable WHERE testtable.id = 2);
COMMIT;
then when two run at once there are several failure modes. One is the already discussed issue with an update re-check. Another is where both UPDATE at the same time, matching zero rows and continuing. Then they both do the EXISTS test, which happens before the INSERT. Both get zero rows, so both do the INSERT. One fails with a duplicate key error.
This is why you need a re-try loop. You might think that you can prevent duplicate key errors or lost updates with clever SQL, but you can't. You need to check row counts or handle duplicate key errors (depending on the chosen approach) and re-try.
Please don't roll your own solution for this. Like with message queuing, it's probably wrong.
Bulk upsert with lock
Sometimes you want to do a bulk upsert, where you have a new data set that you want to merge into an older existing data set. This is vastly more efficient than individual row upserts and should be preferred whenever practical.
In this case, you typically follow the following process:
CREATE a TEMPORARY table
COPY or bulk-insert the new data into the temp table
LOCK the target table IN EXCLUSIVE MODE. This permits other transactions to SELECT, but not make any changes to the table.
Do an UPDATE ... FROM of existing records using the values in the temp table;
Do an INSERT of rows that don't already exist in the target table;
COMMIT, releasing the lock.
For example, for the example given in the question, using multi-valued INSERT to populate the temp table:
BEGIN;
CREATE TEMPORARY TABLE newvals(id integer, somedata text);
INSERT INTO newvals(id, somedata) VALUES (2, 'Joe'), (3, 'Alan');
LOCK TABLE testtable IN EXCLUSIVE MODE;
UPDATE testtable
SET somedata = newvals.somedata
FROM newvals
WHERE newvals.id = testtable.id;
INSERT INTO testtable
SELECT newvals.id, newvals.somedata
FROM newvals
LEFT OUTER JOIN testtable ON (testtable.id = newvals.id)
WHERE testtable.id IS NULL;
COMMIT;
Related reading
UPSERT wiki page
UPSERTisms in Postgres
Insert, on duplicate update in PostgreSQL?
http://petereisentraut.blogspot.com/2010/05/merge-syntax.html
Upsert with a transaction
Is SELECT or INSERT in a function prone to race conditions?
SQL MERGE on the PostgreSQL wiki
Most idiomatic way to implement UPSERT in Postgresql nowadays
What about MERGE?
SQL-standard MERGE actually has poorly defined concurrency semantics and is not suitable for upserting without locking a table first.
It's a really useful OLAP statement for data merging, but it's not actually a useful solution for concurrency-safe upsert. There's lots of advice to people using other DBMSes to use MERGE for upserts, but it's actually wrong.
Other DBs:
INSERT ... ON DUPLICATE KEY UPDATE in MySQL
MERGE from MS SQL Server (but see above about MERGE problems)
MERGE from Oracle (but see above about MERGE problems)

Here are some examples for insert ... on conflict ... (pg 9.5+) :
Insert, on conflict - do nothing.
insert into dummy(id, name, size) values(1, 'new_name', 3)
on conflict do nothing;`
Insert, on conflict - do update, specify conflict target via column.
insert into dummy(id, name, size) values(1, 'new_name', 3)
on conflict(id)
do update set name = 'new_name', size = 3;
Insert, on conflict - do update, specify conflict target via constraint name.
insert into dummy(id, name, size) values(1, 'new_name', 3)
on conflict on constraint dummy_pkey
do update set name = 'new_name', size = 4;

I am trying to contribute with another solution for the single insertion problem with the pre-9.5 versions of PostgreSQL. The idea is simply to try to perform first the insertion, and in case the record is already present, to update it:
do $$
begin
insert into testtable(id, somedata) values(2,'Joe');
exception when unique_violation then
update testtable set somedata = 'Joe' where id = 2;
end $$;
Note that this solution can be applied only if there are no deletions of rows of the table.
I do not know about the efficiency of this solution, but it seems to me reasonable enough.

SQLAlchemy upsert for Postgres >=9.5
Since the large post above covers many different SQL approaches for Postgres versions (not only non-9.5 as in the question), I would like to add how to do it in SQLAlchemy if you are using Postgres 9.5. Instead of implementing your own upsert, you can also use SQLAlchemy's functions (which were added in SQLAlchemy 1.1). Personally, I would recommend using these, if possible. Not only because of convenience, but also because it lets PostgreSQL handle any race conditions that might occur.
Cross-posting from another answer I gave yesterday (https://stackoverflow.com/a/44395983/2156909)
SQLAlchemy supports ON CONFLICT now with two methods on_conflict_do_update() and on_conflict_do_nothing():
Copying from the documentation:
from sqlalchemy.dialects.postgresql import insert
stmt = insert(my_table).values(user_email='a#b.com', data='inserted data')
stmt = stmt.on_conflict_do_update(
index_elements=[my_table.c.user_email],
index_where=my_table.c.user_email.like('%#gmail.com'),
set_=dict(data=stmt.excluded.data)
)
conn.execute(stmt)
http://docs.sqlalchemy.org/en/latest/dialects/postgresql.html?highlight=conflict#insert-on-conflict-upsert

MERGE in PostgreSQL v. 15
Since PostgreSQL v. 15, is possible to use MERGE command. It actually has been presented as the first of the main improvements of this new version.
It uses a WHEN MATCHED / WHEN NOT MATCHED conditional in order to choose the behaviour when there is an existing row with same criteria.
It is even better than standard UPSERT, as the new feature gives full control to INSERT, UPDATE or DELETE rows in bulk.
MERGE INTO customer_account ca
USING recent_transactions t
ON t.customer_id = ca.customer_id
WHEN MATCHED THEN
UPDATE SET balance = balance + transaction_value
WHEN NOT MATCHED THEN
INSERT (customer_id, balance)
VALUES (t.customer_id, t.transaction_value)

WITH UPD AS (UPDATE TEST_TABLE SET SOME_DATA = 'Joe' WHERE ID = 2
RETURNING ID),
INS AS (SELECT '2', 'Joe' WHERE NOT EXISTS (SELECT * FROM UPD))
INSERT INTO TEST_TABLE(ID, SOME_DATA) SELECT * FROM INS
Tested on Postgresql 9.3

Since this question was closed, I'm posting here for how you do it using SQLAlchemy. Via recursion, it retries a bulk insert or update to combat race conditions and validation errors.
First the imports
import itertools as it
from functools import partial
from operator import itemgetter
from sqlalchemy.exc import IntegrityError
from app import session
from models import Posts
Now a couple helper functions
def chunk(content, chunksize=None):
"""Groups data into chunks each with (at most) `chunksize` items.
https://stackoverflow.com/a/22919323/408556
"""
if chunksize:
i = iter(content)
generator = (list(it.islice(i, chunksize)) for _ in it.count())
else:
generator = iter([content])
return it.takewhile(bool, generator)
def gen_resources(records):
"""Yields a dictionary if the record's id already exists, a row object
otherwise.
"""
ids = {item[0] for item in session.query(Posts.id)}
for record in records:
is_row = hasattr(record, 'to_dict')
if is_row and record.id in ids:
# It's a row but the id already exists, so we need to convert it
# to a dict that updates the existing record. Since it is duplicate,
# also yield True
yield record.to_dict(), True
elif is_row:
# It's a row and the id doesn't exist, so no conversion needed.
# Since it's not a duplicate, also yield False
yield record, False
elif record['id'] in ids:
# It's a dict and the id already exists, so no conversion needed.
# Since it is duplicate, also yield True
yield record, True
else:
# It's a dict and the id doesn't exist, so we need to convert it.
# Since it's not a duplicate, also yield False
yield Posts(**record), False
And finally the upsert function
def upsert(data, chunksize=None):
for records in chunk(data, chunksize):
resources = gen_resources(records)
sorted_resources = sorted(resources, key=itemgetter(1))
for dupe, group in it.groupby(sorted_resources, itemgetter(1)):
items = [g[0] for g in group]
if dupe:
_upsert = partial(session.bulk_update_mappings, Posts)
else:
_upsert = session.add_all
try:
_upsert(items)
session.commit()
except IntegrityError:
# A record was added or deleted after we checked, so retry
#
# modify accordingly by adding additional exceptions, e.g.,
# except (IntegrityError, ValidationError, ValueError)
db.session.rollback()
upsert(items)
except Exception as e:
# Some other error occurred so reduce chunksize to isolate the
# offending row(s)
db.session.rollback()
num_items = len(items)
if num_items > 1:
upsert(items, num_items // 2)
else:
print('Error adding record {}'.format(items[0]))
Here's how you use it
>>> data = [
... {'id': 1, 'text': 'updated post1'},
... {'id': 5, 'text': 'updated post5'},
... {'id': 1000, 'text': 'new post1000'}]
...
>>> upsert(data)
The advantage this has over bulk_save_objects is that it can handle relationships, error checking, etc on insert (unlike bulk operations).