Resolve the error by deleting the referenced constraints.įor more examples, see ALTER TABLE (Transact-SQL). If the column contains constraints or other dependencies, an error message will be returned. ALTER TABLE dbo.doc_exb DROP COLUMN column_b The following example shows you how to delete a column. You can delete columns using Transact-SQL in SSMS, Azure Data Studio, or command-line tools such as the sqlcmd utility. If the column participates in a relationship (FOREIGN KEY or PRIMARY KEY), a message prompts you to confirm the deletion of the selected columns and their relationships.Right-click the column you want to delete and choose Delete Column from the shortcut menu.In Object Explorer, right-click the table from which you want to delete columns and choose Design.The following steps explain how to delete columns with Table Designer in SSMS: Resolve the error by deleting the referenced constraints. If the column contains constraints or other dependencies, an error message will display in the Delete Object dialog box. The word COLUMN is optional and can be omitted. For more information, see Section 13.1.18, CREATE TABLE Statement. columndefinition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. Right-click the column that you want to delete, and choose Delete. The syntax for many of the permissible alterations is similar to clauses of the CREATE TABLE statement.In Object Explorer, locate the table from which you want to delete columns, and expand to expose the column names.In Object Explorer, connect to an instance of Database Engine.The following steps explain how to delete columns with Object Explorer in SSMS: When using Object Explorer or Transact-SQL, you must first remove all dependencies on the column. You can't delete a column that has PRIMARY KEY or FOREIGN KEY constraints or other dependencies except when using the Table Designer in SSMS. You can't delete a column that has a CHECK constraint. GitHub: _posts/-mysql-drop-column-if-exists.When you delete a column from a table, the column and all the data it contains are deleted. 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 nested tags are present, the. However, because you need an IF statement, it will need to be a stored procedure. To drop several columns, specify the tableName as an attribute, and then specify a set of nested tags.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. The most common operation you will want to do is to drop a table or column, but only if it exists. 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). Why might you want to code idempotent schema updates? Say your initial version of the schema update got it slightly wrong. 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. You can also use DROP TABLE command to delete complete table but it would remove complete table structure form the database and you would need to re-create this table once again if you wish you store some data. When updating a database schema, it's very useful to make your SQL scripts idempotent. The SQL TRUNCATE TABLE command is used to delete complete data from an existing table. Idempotence describes the property of operations in mathematics and computer science that means that multiple applications of the operation do not change the result.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |