Most databases allow defining UNIQUE key (unique field) that is not a PRIMARY KEY, but DynamoDB does not seem to support unique key definitions.
For example, a model SampleModel defines an id field as a PRIMARY KEY (id = UnicodeAttribute(hash_key=True)). What if another field (let's say name) must also be defined as unique? Given that DynamoDB does not offer unique field specification, and only one PK (hash_key=True) is allowed - how can name be defined as UNIQUE?
As you've already seen, there's no direct support for that in DynamoDB.
If this name attribute is immutable after you write it, you could do the following:
You can create a second DynamoDB table that only has a Partition Key (Partition Key = Primary Key) and stores each name there. When you add an item to the first table, you use a transaction and have a separate insert into the second table that has a condition of the key not already existing. If this transaction fails, you were trying to put an item whose name already existed in the table. If the transaction goes through, you've inserted a new unique name.
This will incur extra cost for the second table and transactions and only works in this narrow use case.
DynamoDB is not built for this pattern, enforce uniqueness in another way.
You create a PK out of whatever value has to be unique (in the same table or second table) and use transactions to enforce it on insert.
https://aws.amazon.com/blogs/database/simulating-amazon-dynamodb-unique-constraints-using-transactions/
I will create three tables. These tables belong to a category name. The 'Department' category linked to the 'Company' category and the 'Department_Unit' category linked to the 'Department' category. The user will first select Company, then select department, then department_unit. Do you think the query I wrote below is correct? Or do I need to write a different and better query?
thankyou very much.
create_table_company= '''CREATE TABLE company(
ID SERIAL PRIMARY KEY ,
NAME VARCHAR NOT NULL ,
); '''
create_table_department = '''CREATE TABLE department (
ID SERIAL PRIMARY KEY ,
NAME VARCHAR NOT NULL ,
company_id BIGINT,
FOREIGN KEY(company_id) REFERENCES COMPANY(id)); '''
create_table_department_unit = '''CREATE TABLE department_unit(
ID SERIAL PRIMARY KEY ,
NAME VARCHAR NOT NULL ,
department_id BIGINT,
FOREIGN KEY(department_id) REFERENCES DEPARTMENT(id)); '''
This data model looks fine.
Don't be worried that you have to join three tables whenever you need the company name that belongs to a department unit: databases are optimized to deal with such joins. In the case of an OLTP workload (you always select only a few rows from a table), this can be handled very efficiently with nested loop joins.
The only thing that is missing from your schema are two indexes:
CREATE INDEX ON department (company_id);
CREATE INDEX ON department_unit (department_id);
These indexes are needed
to make such nested loop joins efficient, for example if you are searching for all units that belong to a department
to make deletes or key updates efficient, which have to search the referencing table to verify that the foreign key constraint is still satisfied
I have created tables in MySQL Workbench as shown below :
ORDRE table:
CREATE TABLE Ordre (
OrdreID INT NOT NULL,
OrdreDato DATE DEFAULT NULL,
KundeID INT DEFAULT NULL,
CONSTRAINT Ordre_pk PRIMARY KEY (OrdreID),
CONSTRAINT Ordre_fk FOREIGN KEY (KundeID) REFERENCES Kunde (KundeID)
)
ENGINE = InnoDB;
PRODUKT table:
CREATE TABLE Produkt (
ProduktID INT NOT NULL,
ProduktBeskrivelse VARCHAR(100) DEFAULT NULL,
ProduktFarge VARCHAR(20) DEFAULT NULL,
Enhetpris INT DEFAULT NULL,
CONSTRAINT Produkt_pk PRIMARY KEY (ProduktID)
)
ENGINE = InnoDB;
and ORDRELINJE table:
CREATE TABLE Ordrelinje (
Ordre INT NOT NULL,
Produkt INT NOT NULL,
AntallBestilt INT DEFAULT NULL,
CONSTRAINT Ordrelinje_pk PRIMARY KEY (Ordre, Produkt),
CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID),
CONSTRAINT Ordrelinje_fk1 FOREIGN KEY (Produkt) REFERENCES Produkt (ProduktID)
)
ENGINE = InnoDB;
so when I try to insert values into ORDRELINJE table i get:
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (srdjank.Ordrelinje, CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID))
I've seen the other posts on this topic, but no luck.
Am I overseeing something or any idea what to do?
Taken from Using FOREIGN KEY Constraints
Foreign key relationships involve a parent table that holds the
central data values, and a child table with identical values pointing
back to its parent. The FOREIGN KEY clause is specified in the child
table.
It will reject any INSERT or UPDATE operation that attempts to create
a foreign key value in a child table if there is no a matching
candidate key value in the parent table.
So your error Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails essentially means that, you are trying to add a row to your Ordrelinje table for which no matching row (OrderID) is present in Ordre table.
You must first insert the row to your Ordre table.
The Problem is with FOREIGN KEY Constraint. By Default (SET FOREIGN_KEY_CHECKS = 1). FOREIGN_KEY_CHECKS option specifies whether or not to check foreign key constraints for InnoDB tables. MySQL - SET FOREIGN_KEY_CHECKS
We can set foreign key check as disable before running Query. Disable Foreign key.
Execute one of these lines before running your query, then you can run your query successfully. :)
1) For Session (recommended)
SET FOREIGN_KEY_CHECKS=0;
2) Globally
SET GLOBAL FOREIGN_KEY_CHECKS=0;
This error generally occurs because we have some values in the referencing field of the child table, which do not exist in the referenced/candidate field of the parent table.
Sometimes, we may get this error when we are applying Foreign Key constraints to existing table(s), having data in them already. Some of the other answers are suggesting to delete the data completely from child table, and then apply the constraint. However, this is not an option when we already have working/production data in the child table. In most scenarios, we will need to update the data in the child table (instead of deleting them).
Now, we can utilize Left Join to find all those rows in the child table, which does not have matching values in the parent table. Following query would be helpful to fetch those non-matching rows:
SELECT child_table.*
FROM child_table
LEFT JOIN parent_table
ON parent_table.referenced_column = child_table.referencing_column
WHERE parent_table.referenced_column IS NULL
Now, you can generally do one (or more) of the following steps to fix the data.
Based on your "business logic", you will need to update/match these unmatching value(s), with the existing values in the parent table. You may sometimes need to set them null as well.
Delete these rows having unmatching values.
Add new rows in your parent table, corresponding to the unmatching values in the child table.
Once the data is fixed, we can apply the Foreign key constraint using ALTER TABLE syntax.
You are getting this constraint check because Ordre table does not have reference OrdreID provided in insert command.
To insert value in Ordrelinje, you first have to enter value in Ordre table and use same OrdreID in Orderlinje table.
Or you can remove not null constraint and insert a NULL value in it.
You must delete data in the child table which does not have any corresponding foreign key value to the parent table primary key .Or delete all data from the child table then insert new data having the same foreign key value as the primary key in the parent table . That should work .
This helped me out after reading #Mr-Faizan's and other answers.
Untick the 'Enable foreign key checks'
in phpMyAdmin and hit the query.
I don't know about WorkBench but the other answers might help you out.
I had the same problem. I was creating relationships on existing tables but had different column values, which were supposed/assumed to be related. For example, I had a table USERS that had a column USERID with rows 1,2,3,4,5. Then I had another child table ORDERS with a column USERID with rows 1,2,3,4,5,6,7. Then I run MySQl command ALTER TABLE ORDERS ADD CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS(USERID) ON DELETE SET NULL ON UPDATE CASCADE;
It was rejected with the message:
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (DBNAME1.#sql-4c73_c0, CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS (USERID) ON DELETE SET NULL ON UPDATE CASCADE)
I exported data from the ORDERS table, then deleted all data from it, re-run the command again, it worked this time, then re-inserted the data with the corresponding USERIDs from the USERS table.
in the foreign key table has a value that is not owned in the primary key table that will be related, so you must delete all data first / adjust the value of your foreign key table according to the value that is in your primary key
I found that changing the foreign key back from not null to null BEFORE I tried to do what I knew was the correct code, got it working. Helped that I was using Mysql workbench. I had to also set SET FOREIGN_KEY_CHECKS=0; and then back to =1; after finished.
This can be fixed by inserting the respective records in the Parent table first and then we can insert records in the Child table's respective column. Also check the data type and size of the column. It should be same as the parent table column,even the engine and collation should also be the same.
TRY THIS! This is how I solved mine. Correct me if am wrong.
In my case the tables were perfectly consistent.
Anyway I was getting this error because I created (by accident) more than one FK constraint on the same field.
I run the following query to show all the keys:
SELECT *
FROM information_schema.table_constraints
WHERE constraint_schema = 'my_db_name'
and I deleted the wrong ones with the following query:
ALTER TABLE my_table
DROP FOREIGN KEY wrong_fk_constraint;
You can check it also running this query:
SHOW CREATE TABLE my_table;
While inserting the foreign key attribute values, first verify the attributes type, as well as primary key attribute value in the parent relation, if the values in parent relation matches, then you can easily insert/update child attribute values.
I was getting this issue even though my parent table had all the values I was referencing in my child table. The issue seemed to be that I could not add multiple child references to a single foreign key. In other words if I had five rows of data referenced the same foreign key, MySQL was only allowing me to upload the first row and giving me the error 1452.
What worked for me was typing the code "SET GLOBAL FOREIGN_KEY_CHECKS=0". After that I closed out of MySQL and then restarted it and I was able to upload all of my data with no errors. I then typed "SET GLOBAL FOREIGN_KEY_CHECKS=1" to set the system back to normal although I'm not entirely sure what FOREIGN_KEY_CHECKS does. Hope this helps!
First allow NULL on the parent table and set the default values to NULL. Next create the foreign key relationship. Afterwards, you can update the values to match accordingly
For PhpMyAdmin , Go to the structure of table where you created foreign key and then click on Relation view , in that choose No Action under on Delete and on Update section.
Note : this work for me.
enter image description here
Your ORDRELINJE table is linked with ORDER table using a foreign key constraint constraint Ordrelinje_fk foreign key(Ordre) references Ordre(OrdreID) according to which Ordre int NOT NULL, column of table ORDRELINJE must match any Ordre int NOT NULL, column of ORDER table.
Now what is happening here is, when you are inserting new row into ORDRELINJE table, according to fk constraint Ordrelinje_fk it is checking ORDER table if the OrdreID is present or not and as it is not matching with any OrderId, Compiler is complaining for foreign key violation. This is the reason you are getting this error.
Foreign key is the primary key of the other table which you use in any table to link between both. This key is bound by the foreign key constraint which you specify while creating the table. Any operation on the data must not violate this constraint. Violation of this constraint may result in errors like this.
Hope I made it clear.
you should insert at least one raw in each tables (the ones you want the foreign keys pointing at) then you can insert or update the values of the foreign keys
you should add data from REFERENCES KEY in PRIMARY TABLE to FOREIGN KEY in CHILD TABLE
it means do not add random data to foreign key ، just use data from primary key that is accessable
description of data in foreign key
The problem occurs because you set the foreign key in child table after you insert some data in the child table.
Try removing all the data from child table, then set the foreign key and afterwards add/insert the data in table, it will work.
check the no. of record in parent table that matches with child table and also primary key must match with foreign key reference.
This works for me.
I am having the same issue here is my scenario
i put empty('') where value is NULL
now this '' value does not match with the parent table's id
here is things need to check , all value with presented in parent table
otherwise remove data from parent table then try
ill squeeze this in here:
my case was trying to create a like for a post which dint exist;
while committing to database the error was raised.
solution was to first create the post then like it.
from my understanding if the post_id was to be saved in the likes table it had to first check with posts table to ascertain existence.
i found it better to have it this way since its more logical to me that way..
When you're using foreign key, your order of columns should be same for insertion.
For example, if you're adding (userid, password) in table1 from table2 then from table2 order should be same (userid, password) and not like (password,userid) where userid is foreign key in table2 of table1.
The answer of your question is that you must set the same value in Primary and secondary key.
Thanks
Actually, i solved just like this "insert into databasename.tablename" it worked. And after when i try to add data like "insert into databasename" it worked to.
Just something else to look for. If you had to delete records from one of your tables and are expecting the values to start at 1, you could get this error. The solution was to run a SHOW * FROM tablename on all the Parent tables. When I did I noticed in one of the tables where I had had a problem earlier and had to delete some records that the primary key values were not what I was expecting them to be from a previous SELECT *.
Probably better answered above, but when working in mysql workbench you don't need to commit the transaction immediatly, you can commit the parent and child element at the same time. So setup the parent with sql or in the gui and add the child in the gui / sql and commit concurrently.
If working in code and getting this issue you can create a factory to create the parent and then create the child / join.
Theoretically you would need an Order to have an OrderId thus create an Order. The create an OrderId and that OrderId may have a number of associated products which you can then add to the OrderId or do with as you wish.
I'm learning how to use sqlite3 with python. The example in the text book I am following is a database where each Country record has a Region, Country, and Population.
The book says:
The following snippet uses the CONSTRAINT keyword to specify
that no two entries in the table being
created will ever have the same values
for region and country:
>>> cur.execute('''
CREATE TABLE PopByCountry(
Region TEXT NOT NULL,
Country TEXT NOT NULL,
Population INTEGER NOT NULL,
CONSTRAINT Country_Key PRIMARY KEY (Region, Country))
''')
Please could you explain what CONSTRAINT Country_Key does here. If I remove it, the PRIMARY KEY statement alone seems to ensure that each country has a unique name for that region.
Country_key is simply giving a name to the constraint. If you do not do this the name will be generated for you. This is useful when there are several constraints on the table and you need to drop one of them.
As an example for dropping the constraint:
ALTER TABLE PopByCountry DROP CONSTRAINT Country_Key
If you omit CONSTRAINT Contry_Key from the statement, SQL server will generate a name for your PRIMARY KEY constraint for you (the PRIMARY KEY is a type of constraint).
By specifically putting CONSTRAINT in the query you are essentially specifying a name for your primary key constraint.