Mysql delete column to table8/31/2023 Note that this syntax is an extension to SQL, and it doesn’t conform to the SQL standard of only having one DROP clause per ALTER TABLE statement. In other RDBMSs (such as MySQL and PostgreSQL), you would need to rewrite DROP COLUMN for each column: ALTER TABLE t1 In SQL Server, you can simply list each column, separated by a comma: ALTER TABLE t1 Some RDBMs allow you to drop multiple columns within a single ALTER TABLE statement. Oracle accepts a CASCADE CONSTRAINTS clause which drops all foreign key constraints that refer to the primary and unique keys defined on the dropped columns as well as all multicolumn constraints defined on the dropped columns. Both keywords are able to be used in MariaDB but they don’t have any effect. In this case, the column was dropped and I got a message explaining that the view called vproducts was also dropped.ĬASCADE and RESTRICT are supported in PostgreSQL, but not in SQL Server or MySQL. Result: NOTICE: drop cascades to view vproducts Here’s what happens if I change the previous example to CASCADE: ALTER TABLE Products Using the CASCADE option will cause any dependent objects to be dropped. Here’s the error I get in PostgreSQL when trying to drop a table that’s referenced by a view: cannot drop column productdescription of table products because other objects depend on it Cascade the Change When using the above statement, if the column has any dependencies, the drop operation will fail, and you’ll get an error. RESTRICT is typically the default behavior, so if you don’t specify any of these arguments, the DBMS will refuse to drop the column if there are any dependent objects. The benefit of doing this is that you won’t get an error if the column doesn’t exist.ĭROP COLUMN IF EXISTS ProductDescription Restrict the Changeĭepending on your RDBMS, you may be able to use the CASCADE and RESTRICT arguments to specify what to do if the column has any dependencies, such as foreign keys or views. The IF EXISTS Argumentĭepending on your RDBMS, you may be able to use the IF EXISTS argument, which conditionally drops the column only if it already exists. This removes the ProductDescription column from the Products table. Some RDBMSs also accept optional CASCADE and RESTRICT arguments, which specify what to do if the column has any dependencies. Some RDBMSs accept an optional IF EXISTS argument which means that it won’t return an error if the column doesn’t exist. The syntax goes like this: ALTER TABLE table_name That removes the column and all its data. Mørch for this advice in the comments.Įxample technique was from Baron Schwartz, originally published at Nabble, paraphrased and extended here.In SQL, if you want to remove a column from a table, you need to use the ALTER TABLE statement with the DROP COLUMN clause. SET optimizer_switch = 'derived_merge=off' Luckily, the optimizer_switch variable can be used to switch off this behaviour although I couldn't recommend doing this as anything more than a short term fix, or for small one-off tasks. However, beware that from MySQL 5.7.6 and onward, the optimiser may optimise out the subquery, and still give you the error. Table, so it doesn't count as the same table you're updating. The nested subquery in the FROM clause creates an implicit temporary Ugly for several reasons, including performance: UPDATE tbl SET col = ( If you absolutely need the subquery, there's a workaround, but it's UPDATE tbl AS aĪlternatively, try nesting the subquery deeper into a from clause. This will cause MySQL to see the table as two different things, allowing destructive changes to go ahead. If the logic is simple enough to re-shape the query, lose the subquery and join the table to itself, employing appropriate selection criteria. Maybe you can just join the table to itself In MySQL, you can't modify the same table which you use in the SELECT part. For a more specific answer about how to best handle the OP's exact query, please see other answers to this question Update: This answer covers the general error classification.
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