How to DROP multiple columns with a single ALTER T

问题:

I would like to write a single SQL command to drop multiple columns from a single table in one ALTER TABLE statement.

From MSDN's ALTER TABLE documentation...

DROP { [CONSTRAINT] constraint_name | COLUMN column_name }

Specifies that constraint_name or column_name is removed from the table. DROP COLUMN is not allowed if the compatibility level is 65 or earlier. Multiple columns and constraints can be listed.

It says that mutliple columns can be listed in the the statement but the syntax doesn't show an optional comma or anything that would even hint at the syntax.

How should I write my SQL to drop multiple columns in one statement (if possible)?

回答1:

For SQL Server:

alter table TableName
    drop column Column1, Column2

The syntax is

DROP { [ CONSTRAINT ] constraint_name | COLUMN column } [ ,...n ] 

For MySQL:

alter table TableName
    drop Column1, drop Column2

回答2:

Summarizing

Oracle:

ALTER TABLE table_name DROP (column_name1, column_name2);

MS SQL:

ALTER TABLE table_name DROP COLUMN column_name1, column_name2

MySql:

ALTER TABLE table_name DROP column_name1, DROP column_name2;

Postgre SQL

ALTER TABLE table_name DROP COLUMN column_name1, DROP COLUMN column_name2;

Be aware

DROP COLUMN does not physically remove the data for some DBMS. E.g. for MS SQL. For fixed length types (int, numeric, float, datetime, uniqueidentifier etc) the space is consumed even for records added after the columns were dropped. To get rid of the wasted space do ALTER TABLE ... REBUILD.

回答3:

create table test (a int, b int , c int, d int);
alter table test drop column b, d;

Be aware that DROP COLUMN does not physically remove the data, and for fixed length types (int, numeric, float, datetime, uniqueidentifier etc) the space is consumed even for records added after the columns were dropped. To get rid of the wasted space do ALTER TABLE ... REBUILD.

回答4:

This may be late, but sharing it for the new users visiting this question. To drop multiple columns actual syntax is

alter table tablename drop column col1, drop column col2 , drop column col3 ....

So for every column you need to specify "drop column" in Mysql 5.0.45.

回答5:

The Syntax as specified by Microsoft for the dropping a column part of an ALTER statement is this

 DROP 
 {
     [ CONSTRAINT ] 
     { 
          constraint_name 
          [ WITH 
           ( <drop_clustered_constraint_option> [ ,...n ] ) 
          ] 
      } [ ,...n ]
      | COLUMN 
      {
          column_name 
      } [ ,...n ]
 } [ ,...n ]

Notice that the [,...n] appears after both the column name and at the end of the whole drop clause. What this means is that there are two ways to delete multiple columns. You can either do this:

ALTER TABLE TableName
    DROP COLUMN Column1, Column2, Column3

or this

ALTER TABLE TableName
    DROP 
        COLUMN Column1,
        COLUMN Column2,
        COLUMN Column3

This second syntax is useful if you want to combine the drop of a column with dropping a constraint:

ALTER TBALE TableName
    DROP
        CONSTRAINT DF_TableName_Column1,
        COLUMN Column1;

When dropping columns SQL Sever does not reclaim the space taken up by the columns dropped. For data types that are stored inline in the rows (int for example) it may even take up space on the new rows added after the alter statement. To get around this you need to create a clustered index on the table or rebuild the clustered index if it already has one. Rebuilding the index can be done with a REBUILD command after modifying the table. But be warned this can be slow on very big tables. For example:

ALTER TABLE Test
    REBUILD;

回答6:

For MySQL (ver 5.6), you cannot do multiple column drop with one single drop-statement but rather multiple drop-statements:

mysql> alter table test2 drop column (c1,c2,c3);
ERROR 1064 (42000): 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 '(c1,c2,c3)' at line 1
mysql> alter table test2 drop column c1,c2,c3;
ERROR 1064 (42000): 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 'c2,c3' at line 1
mysql> alter table test2 drop column c1, drop column c2, drop c3;
Query OK, 0 rows affected (0.64 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> 

BTW, drop <col_name> is shorthanded for drop column <col_name> as you can see from drop c3 above.

回答7:

If it is just single column to delete the below syntax works

ALTER TABLE tablename DROP COLUMN column1;

For deleting multiple columns, using the DROP COLUMN doesnot work, the below syntax works

ALTER TABLE tablename DROP (column1, column2, column3......);

回答8:

alter table tablename drop (column1, column2, column3......);

回答9:

Generic:

ALTER TABLE table_name 
DROP COLUMN column1,column2,column3;

E.g:

ALTER TABLE Student 
DROP COLUMN Name, Number, City;

回答10:

this query will alter the multiple column test it.

create table test(a int,B int,C int);

alter table test drop(a,B);

回答11:

ALTER table table_name Drop column column1, Drop column column2,Drop column column3;

for MySQL DB.

Or you can add some column while altering in the same line:

ALTER table table_name Drop column column1, ADD column column2 AFTER column7;

回答12:

Try following queries:

alter table table_name add field_name data_type

or

alter table table_name drop column field_name

or

alter table table_name drop field_name