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Alter database drop column mysql11/10/2023 GitHub: _posts/-mysql-drop-column-if-exists. If exists (select * from information_lumns where table_name = 'table1' and column_name = 'column2') thenĪlter table table1 drop column `column2` Īlter table table1 add column `column1` varchar(255) NULL Īlter table table1 add column `column2` varchar(255) NULL If exists (select * from information_lumns where table_name = 'table1' and column_name = 'column1') thenĪlter table table1 drop column `column1` If we want to remove any table column in the database table, we will implement the DROP COLUMN command with an ALTER statement. However, because you need an IF statement, it will need to be a stored procedure. But you can fake it, at least in MySQL 5 or later, by querying the database meta-data do see if the column exists, and drop it if it does. MySQL has a built-in modifier for this.įor some reason, the same facility does not exist in MySQL for dropping a column if it exists. 'if you restore the primary key, you sure may revert it back to AUTOINCREMENT'. then alter table table1 drop column column1 end if if exists (select from lumns where table. In case you have composite primary key, do like this- ALTER TABLE tablename DROP PRIMARY KEY,ADD PRIMARY KEY (colname1, colname2) To add primary key in the column. The most common operation you will want to do is to drop a table or column, but only if it exists. But you can fake it, at least in MySQL 5 or later, by querying the database meta-data do see if the column exists, and drop it if it does. You could add yet another schema update to fix this, or if your script was idempotent, you could simply modify the original script and run it again. You actually want a column to be varchar(255), not a varchar(50). A foreign key constraint is defined on the child table. Why might you want to code idempotent schema updates? Say your initial version of the schema update got it slightly wrong. A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. you should be able to run the script more than once, no run should error out, and the end result should be the same as when you ran it the first time. When updating a database schema, it's very useful to make your SQL scripts idempotent. Idempotence describes the property of operations in mathematics and computer science that means that multiple applications of the operation do not change the result.
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