Mysql: Renaming an InnoDB table without updating foreign key references to it?

I am trying to replace an InnoDB table with a new table, and I want all foreign key references that point to the old table to point to the new table.

So I tried this:

SET foreign_key_checks = 0;
ALTER TABLE foo RENAME foo_old;
ALTER TABLE foo_new RENAME foo;

Unfortunately, even with foreign_key_checks disabled, all references pointing to foo are changed to point to foo_old. Now I am looking for either

• a way to change the foreign key references back without rebuilding the entire table, OR
• a way to rename a table without updating foreign key references.

I tried dropping the foreign keys and recreating them, but since the tables are huge, it takes hours. The whole point of replacing the table was to make a schema change with limited downtime.

 Answers

Old questions, but following is a possible way around. Basically move the data rather than renaming the tables. You need to of course make sure the new data adhere to the foreign key rules.

SET foreign_key_checks = 0;
CREATE TABLE IF NOT EXISTS foo_old LIKE foo;
INSERT INTO foo_old SELECT * FROM foo;
TRUNCATE foo;
INSERT INTO foo SELECT * FROM foo_new;

Make sure you run it as one query so the foreign_key_checks applies to the whole thing. Hope this helps.

---

On MySQL 5.6 with innodb_file_per_table=ON allows you to swap the table spaces on the fly. This can't be done completely using SQL as file operations need to be performed separately. First prepare the foo_new table to be copied and drop the foo data:

SET foreign_key_checks = 0;
ALTER TABLE foo DISCARD TABLESPACE;
FLUSH TABLES foo_new FOR EXPORT;

At this point you need to copy the relevant InnoDB files to correct name. Files are stored in your data directory. On Debian, for example, they are by default in /var/lib/mysql/yourdatabase and files are foo_new.ibd, foo_new.cfg and foo_new.frm. Copy them to foo.ibd, foo.cfg and foo.frm, respectively. For example:

$ cp foo_new.ibd foo.ibd
$ cp foo_new.frm foo.frm
$ cp foo_new.cfg foo.cfg

Pay attention that MySQL has access to the new files (e.g. they have correct owner, access rights). Once done you can import the table again and enable foreign keys:

UNLOCK TABLES;
ALTER TABLE foo IMPORT TABLESPACE;
SET foreign_key_checks = 1;

This only copies foo_new to foo. Repeat the steps if you need to copy foo to foo_old.

---

I found a way around this... you just drop the source table instead of renaming it.

For this example, we'll call the table 'mytbl'.

1. create copy of the source table, e.g. 'mytbl_new'
2. copy data into the new table
3. drop the source table 'mytbl'
4. rename 'mytbl_new' to 'mytbl'

The only downside is you can't keep a backup of your original table, but you could mysqldump it before hand. Alternatively, you could create an additional table copy if you wanted a verbatim copy of the original table.

Mysql: How do I see all foreign keys to a table or column?

In MySQL, how do I get a list of all foreign key constraints pointing to a particular table? a particular column? 

 Answers

For a Table:

SELECT 
  TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME
FROM
  INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
  REFERENCED_TABLE_SCHEMA = '<database>' AND
  REFERENCED_TABLE_NAME = '<table>';

For a Column:

SELECT 
  TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME
FROM
  INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
  REFERENCED_TABLE_SCHEMA = '<database>' AND
  REFERENCED_COLUMN_NAME = '<column>';

Basically, we changed REFERENCED_TABLE_NAME with REFERENCED_COLUMN_NAME in the where clause.

---

If you use InnoDB and defined FK's you could query the information_schema database e.g.:

SELECT * FROM information_schema.TABLE_CONSTRAINTS 
WHERE information_schema.TABLE_CONSTRAINTS.CONSTRAINT_TYPE = 'FOREIGN KEY' 
AND information_schema.TABLE_CONSTRAINTS.TABLE_SCHEMA = 'myschema'
AND information_schema.TABLE_CONSTRAINTS.TABLE_NAME = 'mytable';

---

As an alternative to Node’s answer, if you use InnoDB and defined FK’s you could query the information_schema database e.g.:

SELECT CONSTRAINT_NAME, TABLE_NAME, REFERENCED_TABLE_NAME
FROM information_schema.REFERENTIAL_CONSTRAINTS
WHERE CONSTRAINT_SCHEMA = '<schema>'
AND TABLE_NAME = '<table>'

for foreign keys from <table>, or

SELECT CONSTRAINT_NAME, TABLE_NAME, REFERENCED_TABLE_NAME
FROM information_schema.REFERENTIAL_CONSTRAINTS
WHERE CONSTRAINT_SCHEMA = '<schema>'
AND REFERENCED_TABLE_NAME = '<table>'

for foreign keys to <table>

You can also get the UPDATE_RULE and DELETE_RULE if you want them.

---

A quick way to list your FKs (Foreign Key references) using the

KEY_COLUMN_USAGE view:

SELECT CONCAT( table_name, '.',
column_name, ' -> ',
referenced_table_name, '.',
referenced_column_name ) AS list_of_fks
FROM information_schema.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_SCHEMA = (your schema name here)
AND REFERENCED_TABLE_NAME is not null
ORDER BY TABLE_NAME, COLUMN_NAME;

This query does assume that the constraints and all referenced and referencing tables are in the same schema.

Add your own comment.

Source: the official mysql manual.

---

The solution I came up with is fragile; it relies on django's naming convention for foreign keys.

USE information_schema;
tee mysql_output
SELECT * FROM TABLE_CONSTRAINTS WHERE CONSTRAINT_TYPE = 'FOREIGN KEY' AND TABLE_SCHEMA = 'database_name';
notee

Then, in the shell,

grep 'refs_tablename_id' mysql_output

---

If you also want to get the name of the foreign key column:

SELECT i.TABLE_SCHEMA, i.TABLE_NAME, 
       i.CONSTRAINT_TYPE, i.CONSTRAINT_NAME, 
       k.COLUMN_NAME, k.REFERENCED_TABLE_NAME, k.REFERENCED_COLUMN_NAME 
  FROM information_schema.TABLE_CONSTRAINTS i 
  LEFT JOIN information_schema.KEY_COLUMN_USAGE k 
       ON i.CONSTRAINT_NAME = k.CONSTRAINT_NAME 
 WHERE i.TABLE_SCHEMA = '<TABLE_NAME>' AND i.CONSTRAINT_TYPE = 'FOREIGN KEY' 
 ORDER BY i.TABLE_NAME;

MySQL: delete a row ignoring foreign key constraint

So I am working on a few tables and there are some data inconsistency between them... One or two tables have a foreign key constraint on a particular table (call it table X), but that table has multiple rows with the foreign key column.

What I want to do is to remove the duplicated rows in table X, but the foreign key constraint is preventing me from doing this. Is there a way to force delete the rows while ignoring the foreign key constraint since I know what I'm doing?

 Answers

SET foreign_key_checks = 0

That will prevent MySQL from checking foreign keys. Make sure to set it back to 1 when you are done though.

Also, you could always drop the foreign key and then add it later if you wanted to only affect a singular key

ALTER TABLE t DROP FOREIGN KEY fk

Mysql: Delete all foreign keys in database

I would like to rename a column in a table that is a foreign key to many tables. 

Apparently this is only possible if you delete the constraints.

I dont want to delete all the constraints manually is there a way to delete all the foreign key constraints in the database?

I have also tried SET FOREIGN_KEY_CHECKS=0; but I still cant rename the column.

 Answers

Executing the following query

select * from information_schema.key_column_usage

will show you all the constraints (with the column name, constraint type, table and schema) that exist in your database. You'll notice these columns:

CONSTRAINT_CATALOG
CONSTRAINT_SCHEMA
CONSTRAINT_NAME
TABLE_CATALOG
TABLE_SCHEMA
TABLE_NAME
COLUMN_NAME
ORDINAL_POSITION
POSITION_IN_UNIQUE_CONSTRAINT
REFERENCED_TABLE_SCHEMA
REFERENCED_TABLE_NAME
REFERENCED_COLUMN_NAME

Then, if you're planning to delete each constraint you have referencing your column, you should consider the REFERENCED_* columns and run something like:

DELETE FROM information_schema.key_column_usage 
WHERE 
    REFERENCED_TABLE_SCHEMA='myschema'
    AND
    REFERENCED_TABLE_NAME='mytable'
    AND
    REFERENCED_COLUMN_NAME='mycolumn'

http://dev.mysql.com/doc/refman/5.1/en/key-column-usage-table.html

---

You Can Try using As Like of Following ..

ALTER TABLE tableName
DROP FOREIGN KEY fieldName;
ADD FOREIGN KEY (newForignKeyFieldName);

Also you can try with Reference Key.As like .....

ALTER TABLE tableName
DROP FOREIGN KEY fieldName;
ADD FOREIGN KEY (newForignKeyFieldName)
REFERENCES anotherTableName(reference_id);

Foreign key referring to primary keys across multiple tables?

I have to two tables namely employees_ce and employees_sn under the database employees.

They both have their respective unique primary key columns.

I have another table called deductions, whose foreign key column I want to reference to primary keys of employees_ce as well as employees_sn. Is this possible?

for example

employees_ce
--------------
empid   name
khce1   prince

employees_sn
----------------
empid   name
khsn1   princess

so is this possible?

deductions
--------------
id      name
khce1   gold
khsn1   silver

 Answers

Assuming that I have understood your scenario correctly, this is what I would call the right way to do this:

Start from a higher-level description of your database! You have employees, and employees can be "ce" employees and "sn" employees (whatever those are). In object-oriented terms, there is a class "employee", with two sub-classes called "ce employee" and "sn employee".

Then you translate this higher-level description to three tables: employees, employees_ce and employees_sn:

• employees(id, name)
• employees_ce(id, ce-specific stuff)
• employees_sn(id, sn-specific stuff)

Since all employees are employees (duh!), every employee will have a row in the employees table. "ce" employees also have a row in the employees_ce table, and "sn" employees also have a row in the employees_sn table. employees_ce.id is a foreign key to employees.id, just as employees_sn.id is.

To refer to an employee of any kind (ce or sn), refer to the employees table. That is, the foreign key you had trouble with should refer to that table!

---

Actually I do this myself. I have a table called 'Comments' which contains comments for records in 3 other tables. Neither solution actually handles everything you probably want it to. In your case, you would do this:

Solution 1:

1. Add a tinyint field to employees_ce and employees_sn that has a default value which is different in each table (This field represents a 'table identifier', so we'll call them tid_ce & tid_sn)
2. Create a Unique Index on each table using the table's PK and the table id field.
3. Add a tinyint field to your 'Deductions' table to store the second half of the foreign key (the Table ID)
4. Create 2 foreign keys in your 'Deductions' table (You can't enforce referential integrity, because either one key will be valid or the other...but never both:

   ALTER TABLE [dbo].[Deductions]  WITH NOCHECK ADD  CONSTRAINT [FK_Deductions_employees_ce] FOREIGN KEY([id], [fk_tid])
   REFERENCES [dbo].[employees_ce] ([empid], [tid])
   NOT FOR REPLICATION 
   GO
   ALTER TABLE [dbo].[Deductions] NOCHECK CONSTRAINT [FK_600_WorkComments_employees_ce]
   GO
   ALTER TABLE [dbo].[Deductions]  WITH NOCHECK ADD  CONSTRAINT [FK_Deductions_employees_sn] FOREIGN KEY([id], [fk_tid])
   REFERENCES [dbo].[employees_sn] ([empid], [tid])
   NOT FOR REPLICATION 
   GO
   ALTER TABLE [dbo].[Deductions] NOCHECK CONSTRAINT [FK_600_WorkComments_employees_sn]
   GO
   
   employees_ce
   --------------
   empid    name     tid
   khce1   prince    1
   
   employees_sn
   ----------------
   empid    name     tid 
   khsn1   princess  2
   
   deductions
   ----------------------
   id      tid       name  
   khce1   1         gold
   khsn1   2         silver         
   ** id + tid creates a unique index **
   

Solution 2: This solution allows referential integrity to be maintained: 1. Create a second foreign key field in 'Deductions' table , allow Null values in both foreign keys, and create normal foreign keys:

    employees_ce
    --------------
    empid   name
    khce1   prince 

    employees_sn
    ----------------
    empid   name     
    khsn1   princess 

    deductions
    ----------------------
    idce    idsn      name  
    khce1   *NULL*    gold
    *NULL*  khsn1     silver         

Integrity is only checked if the column is not null, so you can maintain referential integrity.

---

Technically possible. You would probably reference employees_ce in deductions and employees_sn. But why don't you merge employees_sn and employees_ce? I see no reason why you have two table. No one to many relationship. And (not in this example) many columns.

If you do two references for one column, an employee must have an entry in both tables.

---

Assuming you must have two tables for the two employee types for some reason, I'll extend on vmarquez's answer:

Schema:

employees_ce (id, name)
employees_sn (id, name)
deductions (id, parentId, parentType, name)

Data in deductions:

deductions table
id      parentId      parentType      name
1       1             ce              gold
2       1             sn              silver
3       2             sn              wood
...

This would allow you to have deductions point to any other table in your schema. This kind of relation isn't supported by database-level constraints, IIRC so you'll have to make sure your App manages the constraint properly (which makes it more cumbersome if you have several different Apps/services hitting the same database).

Mysql: Add Foreign Key to existing table

I want to add a Foreign Key to a table called "katalog".

ALTER TABLE katalog 
ADD CONSTRAINT `fk_katalog_sprache` 
FOREIGN KEY (`Sprache`)
REFERENCES `Sprache` (`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL;

When I try to do this, I get this error message:

Error Code: 1005. Can't create table 'mytable.#sql-7fb1_7d3a' (errno: 150)

Error in INNODB Status:

120405 14:02:57 Error in foreign key constraint of table mytable.#sql-7fb1_7d3a:

FOREIGN KEY (`Sprache`)
REFERENCES `Sprache` (`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL:
Cannot resolve table name close to:
(`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL

When i use this query it works, but with wrong "on delete" action:

ALTER TABLE `katalog` 
ADD FOREIGN KEY (`Sprache` ) REFERENCES `sprache` (`ID` )

Both tables are InnoDB and both fields are "INT(11) not null". I'm using MySQL 5.1.61. Trying to fire this ALTER Query with MySQL Workbench (newest) on a MacBook Pro.

Table Create Statements:

CREATE TABLE `katalog` (
`ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
`Name` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
`AnzahlSeiten` int(4) unsigned NOT NULL,
`Sprache` int(11) NOT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `katalogname_uq` (`Name`)
 ) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci ROW_FORMAT=DYNAMIC$$

CREATE TABLE `sprache` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
 `Bezeichnung` varchar(45) NOT NULL,
 PRIMARY KEY (`ID`),
 UNIQUE KEY `Bezeichnung_UNIQUE` (`Bezeichnung`),
KEY `ix_sprache_id` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

 Answers

To add a foreign key (grade_id) to an existing table (users), follow the following steps:

ALTER TABLE users ADD grade_id SMALLINT UNSIGNED NOT NULL DEFAULT 0;
ALTER TABLE users ADD CONSTRAINT fk_grade_id FOREIGN KEY (grade_id) REFERENCES grades(id);

---

check this link. It has helped me with errno 150: http://verysimple.com/2006/10/22/mysql-error-number-1005-cant-create-table-mydbsql-328_45frm-errno-150/

On the top of my head two things come to mind.

• Is your foreign key index a unique name in the whole database (#3 in the list)?
• Are you trying to set the table PK to NULL on update (#5 in the list)?

I'm guessing the problem is with the set NULL on update (if my brains aren't on backwards today as they so often are...).

Edit: I missed the comments on your original post. Unsigned/not unsigned int columns maybe resolved your case. Hope my link helps someone in the future thought.

---

Simple Steps...

ALTER TABLE t_name1 ADD FOREIGN KEY (column_name) REFERENCES t_name2(column_name)

---

MySQL will execute this query:

ALTER TABLE `db`.`table1`
ADD COLUMN `col_table2_fk` INT UNSIGNED NULL,
ADD INDEX `col_table2_fk_idx` (`col_table2_fk` ASC),
ADD CONSTRAINT `col_table2_fk1`
FOREIGN KEY (`col_table2_fk`)
REFERENCES `db`.`table2` (`table2_id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION;

Mysql: Foreign key constraints: When to use ON UPATE and ON DELETE

I'm designing my database schema using MySQL Workbench, which is pretty cool because you can do diagrams and it converts them :P

Anyways, I've decided to use InnoDB because of it's Foreign Key support. One thing I noticed though is that it allows you to set On Update and on Delete options for foreign keys. Can someone explain where "Restrict", "Cascade" and set null could be used in a simple example?

For example, say I have a user table which includes a userID. And say I have a message table message which is a many-to-many which has 2 foreign keys (which reference the same primary key, userID in the user table). Is setting the On Update and On Delete options any useful in this case? If so, which one do I choose? If this isn't a good example, could you please come up with a good example to illustrate how these could be useful?

Answers

Do not hesitate to put constraints on the database. You'll be sure to have a consistent database, and that's one of the good reasons to use a database. Especially if you have several applications requesting it (or just one application but with a direct mode and a batch mode using different sources).

With MySQL you do not have advanced constraints like you would have in postgreSQL but at least the foreign key constraints are quite advanced.

We'll take an example, a company table with a user table containing people from theses company

CREATE TABLE COMPANY (
     company_id INT NOT NULL,
     company_name VARCHAR(50),
     PRIMARY KEY (company_id)
) ENGINE=INNODB;

CREATE TABLE USER (
     user_id INT, 
     user_name VARCHAR(50), 
     company_id INT,
     INDEX company_id_idx (company_id),
     FOREIGN KEY (company_id) REFERENCES COMPANY (company_id) ON...
) ENGINE=INNODB;

Let's look at the ON UPDATE clause:

• ON UPDATE RESTRICT : the default : if you try to update a company_id in table COMPANY the engine will reject the operation if one USER at least links on this company.
• ON UPDATE NO ACTION : same as RESTRICT.
• ON UPDATE CASCADE : the best one usually : if you update a company_id in a row of table COMPANY the engine will update it accordingly on all USER rows referencing this COMPANY (but no triggers activated on USER table, warning). The engine will track the changes for you, it's good.
• ON UPDATE SET NULL : if you update a company_id in a row of table COMPANY the engine will set related USERs company_id to NULL (should be available in USER company_id field). I cannot see any interesting thing to do with that on an update, but I may be wrong.

And now on the ON DELETE side:

• ON DELETE RESTRICT : the default : if you try to delete a company_id Id in table COMPANY the engine will reject the operation if one USER at least links on this company, can save your life.
• ON DELETE NO ACTION : same as RESTRICT
• ON DELETE CASCADE : dangerous : if you delete a company row in table COMPANY the engine will delete as well the related USERs. This is dangerous but can be used to make automatic cleanups on secondary tables (so it can be something you want, but quite certainly not for a COMPANY<->USER example)
• ON DELETE SET NULL : handful : if you delete a COMPANY row the related USERs will automatically have the relationship to NULL. If Null is your value for users with no company this can be a good behavior, for example maybe you need to keep the users in your application, as authors of some content, but removing the company is not a problem for you.

usually my default is: ON DELETE RESTRICT ON UPDATE CASCADE. with some ON DELETE CASCADE for track tables (logs--not all logs--, things like that) and ON DELETE SET NULL when the master table is a 'simple attribute' for the table containing the foreign key, like a JOB table for the USER table.

Edit

It's been a long time since I wrote that. Now I think I should add one important warning. MySQL has one big documented limitation with cascades. Cascades are not firing triggers. So if you were over confident enough in that engine to use triggers you should avoid cascades constraints.

• http://dev.mysql.com/doc/refman/5.6/en/triggers.html

MySQL triggers activate only for changes made to tables by SQL statements. They do not activate for changes in views, nor by changes to tables made by APIs that do not transmit SQL statements to the MySQL Server

• http://dev.mysql.com/doc/refman/5.6/en/stored-program-restrictions.html#stored-routines-trigger-restrictions

==> See below the last edit, things are moving on this domain

Triggers are not activated by foreign key actions.

And I do not think this will get fixed one day. Foreign key constraints are managed by the InnoDb storage and Triggers are managed by the MySQL SQL engine. Both are separated. Innodb is the only storage with constraint management, maybe they'll add triggers directly in the storage engine one day, maybe not.

But I have my own opinion on which element you should choose between the poor trigger implementation and the very useful foreign keys constraints support. And once you'll get used to database consistency you'll love PostgreSQL.

12/2017-Updating this Edit about MySQL:

as stated by @IstiaqueAhmed in the comments, the situation has changed on this subject. So follow the link and check the real up-to-date situation (which may change again in the future).

---

Addition to @MarkR answer - one thing to note would be that many PHP frameworks with ORMs would not recognize or use advanced DB setup (foreign keys, cascading delete, unique constraints), and this may result in unexpected behaviour.

For example if you delete a record using ORM, and your DELETE CASCADE will delete records in related tables, ORM's attempt to delete these related records (often automatic) will result in error.