How to rename a field used as a foreign key in MySQL?
I found a workaround, using phpMyAdmin:
Remove the constraint
Rename the field
Re-add the constraint on the field (the same field but renamed)
But maybe there is a way to do that in one SQL request...Any idea?
You could execute the following code, and make the necessary changes according to your column and table names.
ALTER TABLE table_name
DROP FOREIGN KEY fk_constraint_name;
ALTER TABLE table_name
CHANGE fk_column_name new_fk_column_name datatype;
ALTER TABLE table_name
ADD FOREIGN KEY fk_constraint_name
REFERENCES parent_table_name(pk_column_name_id);
Let me know how this works for you.
Related
In mysql, when creating a table, you can append:
unique(username)
but i dont know how to alter a table to add that containt.
I was trying:
alter table Company constraint unique (username)
I mixed up the syntax.
alter table Company add constraint unique (username);
but this will fail if there are duplicates... So either drop and recreate if it is a sample db, or delete out duplicates.
I have had a 1 to many relationship between course and instructor, which I wanted to drop. When I tried to drop the instructorID in course table it told me that. I couldn't drop it as it was a foreign key. Then I decided to drop it like this:
ALTER TABLE course DROP FOREIGN KEY instructorID
But i get this error :
#1091 - Can't DROP 'InstructorID'; check that column/key exists
I don't get what this error means. what am i doing wrong?
Please run an SHOW CREATE TABLE course; to make sure instructorID is the name of foreign key constraint.
Additional:
The error means MySQL searches for a foreign key constraint named "InstructorID" but there is no constraint with such name, maybe this is your column name, but you have to use the constraint name to delete foreign keys.
After you run SHOW CREATE table course;
you should find the fk symbol which is commonly like the one bellow:
(course_ibfk_1)
it may differ according to your mysql version you are using then drop the foreign key using the fk symbol as follow :
alter table course drop foreign key course_ibfk_1;
You need to delete the 'foreign key constraint' and the 'key'.
Alter Table <table name> drop foreign key <constraint_name>
Alter table <table name> drop key <column name>
If any of you still not able to DROP the table. Try this. You can able to see all the details by running this
SELECT
TABLE_NAME,
COLUMN_NAME,
CONSTRAINT_NAME,
REFERENCED_TABLE_NAME,
REFERENCED_COLUMN_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME = 'ReferenceTableName'; <-- change only this
If you want to see just the constrains
SELECT
CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME = 'ReferenceTableName';
To drop a FOREIGN KEY constraint:
MySQL:
ALTER TABLE Orders DROP FOREIGN KEY {Constraint/Key_name};
For SQL Server / Oracle / MS Access:
ALTER TABLE Orders DROP CONSTRAINT {Constraint/Key_name};
the reason why you cannot drop InstructorID is because you need to use the name of the constraint of the Foreign key . KevDev specified that you must run 'SHOW CREATE TABLE course' to find the constraint name. after doing so , you can delete the foreign key. BUT wait theres more, the 'key' still stays behind which must get deleted. You can run 'SHOW CREATE TABLE' course to check that the key is still behind. once checking that it is still there then perform what Bobby advised. 'Alter table drop key' in doing so you have fully removed the the foreign key
Can't DROP 'string'; check that column/key exists: ALTER TABLE accreditor_architectures DROP string error show in terminal when remove column from data base in ruby on rails
I want to remove constraints from my table. My query is:
ALTER TABLE `tbl_magazine_issue`
DROP CONSTRAINT `FK_tbl_magazine_issue_mst_users`
But I got an error:
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'constraint FK_tbl_magazine_issue_mst_users' at line 1
Mysql has a special syntax for dropping foreign key constraints:
ALTER TABLE tbl_magazine_issue
DROP FOREIGN KEY FK_tbl_magazine_issue_mst_users
I had the same problem and I got to solve with this code:
ALTER TABLE `table_name` DROP FOREIGN KEY `id_name_fk`;
ALTER TABLE `table_name` DROP INDEX `id_name_fk`;
There is no such thing as DROP CONSTRAINT in MySQL. In your case you could use DROP FOREIGN KEY instead.
If the constraint is not a foreign key, eg. one added using 'UNIQUE CONSTRAINT (colA, colB)' then it is an index that can be dropped using ALTER TABLE ... DROP INDEX ...
To add a little to Robert Knight's answer, since the title of the post itself doesn't mention foreign keys (and since his doesn't have complete code samples and since SO's comment code blocks don't show as well as the answers' code blocks), I'll add this for unique constraints. Either of these work to drop the constraint:
ALTER TABLE `table_name` DROP KEY `uc_name`;
or
ALTER TABLE `table_name` DROP INDEX `uc_name`;
Also nice, you can temporarily disable all foreign key checks from a mysql database:
SET FOREIGN_KEY_CHECKS=0;
And to enable it again:
SET FOREIGN_KEY_CHECKS=1;
Some ORM's or frameworks use a different naming convention for foreign keys than the default FK_[parent table]_[referenced table]_[referencing field], because they can be altered.
Laravel for example uses [parent table]_[referencing field]_foreign as naming convention. You can show the names of the foreign keys by using this query, as shown here:
SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_SCHEMA = '<database>' AND REFERENCED_TABLE_NAME = '<table>';
Then remove the foreign key by running the before mentioned DROP FOREIGN KEY query and its proper name.
The simplest way to remove constraint is to use syntax ALTER TABLE tbl_name DROP CONSTRAINT symbol; introduced in MySQL 8.0.19:
As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax for dropping and altering existing constraints of any type, where the constraint type is determined from the constraint name
ALTER TABLE tbl_magazine_issue DROP CONSTRAINT FK_tbl_magazine_issue_mst_users;
db<>fiddle demo
For those that come here using MariaDB:
Note that MariaDB allows DROP CONSTRAINT statements in general, for example for dropping check constraints:
ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
https://mariadb.com/kb/en/library/alter-table/
Go to structure view of the table
You will see 2 option at top a.Table structure b.Relation view.
Now click on Relation view , here you can drop your foreign key constraint. You will get all relation here.
In MySQL you have to specify what kind of constraint you want to remove:
Example:
CONSTRAINT `shop_ibfk_1` FOREIGN KEY (`fb_user_id`) REFERENCES `fb_user` (`id`),
CONSTRAINT `shop_chk_1` CHECK ((`import_lock` in (0,1)))
the first one you would remove with:
alter mytable shop drop FOREIGN KEY `shop_ibfk_1`;
the second one with
alter mytable drop CHECK `shop_chk_1`;
this will works on MySQL to drop constraints
alter table tablename drop primary key;
alter table tablename drop foreign key;
How can I drop the "Unique Key Constraint" on a column of a MySQL table using phpMyAdmin?
A unique constraint is also an index.
First use SHOW INDEX FROM tbl_name to find out the name of the index. The name of the index is stored in the column called key_name in the results of that query.
Then you can use DROP INDEX:
DROP INDEX index_name ON tbl_name
or the ALTER TABLE syntax:
ALTER TABLE tbl_name DROP INDEX index_name
You can DROP a unique constraint from a table using phpMyAdmin as requested as shown in the table below. A unique constraint has been placed on the Wingspan field. The name of the constraint is the same as the field name, in this instance.
The indexes capable of placing a unique key constraint on a table are PRIMARY and UNIQUE indexes.
To remove the unique key constraint on a column but keep the index, you could remove and recreate the index with type INDEX.
Note that it is a good idea for all tables to have an index marked PRIMARY.
To add UNIQUE constraint using phpmyadmin, go to the structure of that table and find below and click that,
To remove the UNIQUE constraint, same way, go to the structure and scroll down till Indexes Tab and find below and click drop,
Hope this works.
Enjoy ;)
If you want to remove unique constraints from MySQL database table, use alter table with drop index.
Example:
CREATE TABLE unique_constraints (
unid INT,
activity_name VARCHAR(100),
CONSTRAINT activty_uqniue UNIQUE (activity_name),
PRIMARY KEY (unid)
);
ALTER TABLE unique_constraints
DROP INDEX activty_uqniue;
Where activty_uqniue is UNIQUE constraint for activity_name column.
For WAMP 3.0 :
Click Structure
Below Add 1 Column you will see '- Indexes'
Click -Indexes and drop whichever index you want.
The constraint could be removed with syntax:
ALTER TABLE
As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax for dropping and altering existing constraints of any type, where the constraint type is determined from the constraint name: ALTER TABLE tbl_name DROP CONSTRAINT symbol;
Example:
CREATE TABLE tab(id INT, CONSTRAINT unq_tab_id UNIQUE(id));
-- checking constraint name if autogenerated
SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE TABLE_NAME = 'tab';
-- dropping constraint
ALTER TABLE tab DROP CONSTRAINT unq_tab_id;
db<>fiddle demo
This might help:
Inside your sql terminal
FIRST STEP:
SHOW INDEX FROM {YOUR_TABLE_NAME}
SECOND STEP:
SHOW INDEX FROM {YOUR_TABLE_NAME} WHERE Column_name='ACTUAL_COLUMN_NAME_YOU_GOT_FROM_FIRST_STEP_OUTPUT'
THIRD STEP:
ORIGINAL_KEY_NAME_VALUE = SECOND_STEP_RESPONSE["Key_name"]
FOURTH STEP:
ALTER TABLE {YOUR_TABLE_NAME} DROP INDEX ${ORIGINAL_KEY_NAME_VALUE}
while dropping unique key we use index
ALTER TABLE tbl
DROP INDEX unique_address;
my table name is buyers which has a unique constraint column emp_id now iam going to drop the emp_id
step 1: exec sp_helpindex buyers, see the image file
step 2: copy the index address
step3: alter table buyers drop constraint [UQ__buyers__1299A860D9793F2E]
alter table buyers
drop column emp_id
note:
Blockquote
instead of buyers change it to your table name :)
Blockquote
thats all column name emp_id with constraints is dropped!
How do I rename a primary key column in MySQL?
it's no different than altering any other column --
ALTER TABLE `pkey` CHANGE `keyfield` `keyfield2` INT( 11 ) NOT NULL AUTO_INCREMENT
this changes the column keyfield in table pkey to be called keyfield2 -- you have to supply the definition afterwards, as usual.
Leave off the PRIMARY KEY part of the alter statement. The primary key will be updated automatically.
Maybe you have a foreign key constraint in place. You can disable those by SET foreign_key_constraints=0 but you have to remember to update the database afterwards.
Possible a bad practice work around. But you could export your entire db to an sql text file. Find and replace the PK you want to rename, and then restore the database over sql.
If you are working with InnoDB then I think you cannot rename primary keys, at least you can't if they are referenced by foreign keys. You need to dump the database, rename the columns and referencing keys in the dump file, then reload the database.
If others tables have a foreign key on your table, you cannot directly rename the column using alter table, it will throw the following error: [HY000][1025] Error on rename of xxx to yyy (errno: 150)
You must :
drop foreign keys from others tables pointing to the primary key you want to rename
rename the primary key
add the foreign column to other tables
When renaming a table in Intellij, it generates you the code do drop and add the foreign key.