如何更改表以使字段组合成为MySQL中的主键？

Question

I created a table named mytable1 in MySQL with 4 fields namely col1, col2, col3, col4,. Initially while creating the table I made col1 the primary key. But now I need to make the combination of (col1, col2,col3) as the primary key as col1's value need to repeat in my table.

Please help.

Answer 1

ALTER TABLE mytable1 DROP PRIMARY KEY, ADD PRIMARY KEY(col1, col2, col3);

Answer 2

You need to drop first the primary since you can only have one primary in a table and add it again. This can be done in a single line,

ALTER TABLE  tableName DROP PRIMARY KEY, ADD PRIMARY KEY (col1, col2, col3)

• SQLFiddle Demo

Answer 3

ALTER TABLE mytable1
    DROP PRIMARY KEY,
    ADD PRIMARY KEY (col1, col2, col3);