Mysql – Handling error of foreign key

foreign keyMySQLmysql-5.5

How to set a FK value NULL upon receiving error for foreign key constraint?

Consider a simple table a

CREATE TABLE test
(
    id int(11) NOT NULL AUTO_INCREMENT
    ex_key int(11),
    FOREIGN KEY(ex_key) REFERENCES ex_table(ex_key) ON DELETE SET NULL,
    PRIMARY KEY (id)
)

If receiving FK error for an invalid ex_key

INSERT INTO test (id, ex_key) VALUES ('22', '22');

we can insert by setting NULL value for the foreign key

INSERT INTO test (id, ex_key) VALUES ('22', NULL);

It can be done by an IF statement, but is there a way to do so in the same query?

I hope to find a solution similar to available ON DUPLICATE KEY UPDATE for UNIQUE columns.

Best Answer

So, when you insert a row into test with an invalid ex_key value (one that isn't in the referenced column in the referenced table), you want that insert to succeed, instead of fail, and you want the invalid value inserted as NULL instead instead of the value you attempted to insert. Right?

A "before insert" trigger should allow you to accomplish that, by doing the lookup and changing the value of ex_key to NULL if ex_key isn't a legit value.

Of course, there's some overhead on processing any trigger, but if you need this functionality, this is probably the most painless way to do it.

Also, If you already have a "before insert" trigger on this table, you would just add the IF ... END IF block to the body of the existing trigger.

DELIMITER //

CREATE TRIGGER test_bi BEFORE INSERT ON test FOR EACH ROW
BEGIN
    IF (SELECT ex_key FROM ex_table WHERE ex_table.ex_key = NEW.ex_key) IS NULL THEN
        SET NEW.ex_key = NULL;
    END IF;
END;

//

DELIMITER ;

I built a proof of concept of this with MySQL 5.5.27, here: http://sqlfiddle.com/#!2/d0a8e/1/0

Note, this also completely prevents the foreign key error from even occurring, because the trigger replaces NEW.ex_key's incoming value with NULL (when appropriate) before the validity of the value is checked against the foreign key constraint by InnoDB.

Related Solutions

MySQL – Foreign Key Constraint Fails

You need to have already declared the table that the foreign key references, before you can define a foreign key that references it.

Once you declare the second table, you can then declare the first table. Tested here on MySQL 5.5.27.

If you need to bypass the validation, you can do this:

SET FOREIGN_KEY_CHECKS = 0;
-- declare tables
SET FOREIGN_KEY_CHECKS = 1;

http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_foreign_key_checks

PostgreSQL – insert/update violates foreign key constraints

There are a few problems with your tables. I'll try to address the foreign keys first, since you question asked about them :)

But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Of course, the definition of Table3 in your second attempt has syntax and logical errors, but the basic idea is:

CREATE TABLE table3 (   
  "ID" bigint NOT NULL DEFAULT '0',   
  "DataID" bigint DEFAULT NULL,   
  "Address" numeric(20) DEFAULT NULL,   
  "Data" bigint DEFAULT NULL,
   PRIMARY KEY ("ID"),   
   FOREIGN KEY ("DataID") REFERENCES Table1("DataID") on delete cascade on update cascade,   
   FOREIGN KEY ("Address") REFERENCES Table2("Address") on delete cascade on update cascade
);

This definition tell PostgreSQL roughly the following: "Create a table with four columns, one will be the primary key (PK), the others can be NULL. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataID˙and Table2 for Address. If there is no such value in those tables, then return an error." This last point which you've seen first:

ERROR: insert or update on table "Table3" violates foreign key constraint 
    "Table3_DataID_fkey" DETAIL: Key (DataID)=(27856) is not present in table "Table1".

So simple: if there is no row in Table1 where DataID = 27856, then you can't insert that row into Table3.

If you need that row, you should first insert a row into Table1 with DataID = 27856, and only then try to insert into Table3. If this seems to you not what you want, please describe in a few sentences what you want to achieve, and we can help with a good design.

And now about the other problems.

You define your PKs as

CREATE all_your_tables (
    first_column NOT NULL DEFAULT '0',   
    [...]
    PRIMARY KEY ("ID"),

A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. If you give a static DEFAULT (like '0') to a UNIQUE column, you will experience bad surprises all the time. This is what you got in your third error message.

Furthermore, '0' means a text string, but not a number (bigint or numeric in your case). Use simply 0 instead (or don't use it at all, as I written above).

And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). At the same time, it is the PK of the table. The column name and the data type suggests that this address can change in the future. If this is true, than it is a very bad choice for a PK. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like

ID        DataID           Address             Data
123       3216547          1234567890454       654897564134569

Now that address happens to change to something other. How do you make your child row in Table3 follow its parent to the new address? (There are solutions for this, but can cause much confusion.) If this is your case, add an ID column to your table, which will not contain any information from the real world, it will simply serve as an identification value (that is, ID) for an address.

Best Answer

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MySQL – Foreign Key Constraint Fails

PostgreSQL – insert/update violates foreign key constraints

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