How to set AUTO_INCREMENT PRIMARY KEY when using 'AS SELECT' - mysql

Question

I am trying to create a temporary table and right now I am doing something like this:
CREATE IF NOT EXISTS TABLE tempdb.student AS (SELECT * FROM student LIMIT 0)
then
ALTER TABLE tempdb.student MODIFY id INT(20) NOT NULL AUTO_INCREMENT PRIMARY KEY
However, I need to find a way to do this without the ALTER statement because both statements are getting ran together and if the table has already been created, I am getting an error because of the multiple primary keys. Any advice? Thanks

Answer 1

You can define columns, indexes, table options, and partitioning before the AS keyword.
CREATE IF NOT EXISTS TABLE tempdb.student (
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(10),
enroll_date DATE,
...etc...
)
AS SELECT * FROM student LIMIT 0
This is the only way you can set some column or table options, such as auto-increment.
But it means you have to define all the columns, not just id.
Since you are using LIMIT 0 it means you don't actually want any rows from the source table. I guess you are just trying to duplicate the table structure, but none of the data.
In that case, you might like to use the following instead:
CREATE IF NOT EXISTS TABLE tempdb.student LIKE student;
This duplicates other column and table options, and indexes, and partitions. The new table will have the AUTO_INCREMENT option for its primary key.

Answer 2

You can explicitly check if the table exists (you need to be careful if the code can be executed in parallel):
SELECT
COUNT(*) INTO #exists
FROM
information_schema.tables
WHERE
table_schema = 'tempdb'
AND table_name = 'student'
;
IF #exists = 0 THEN
CREATE TABLE tempdb.student AS (SELECT * FROM student LIMIT 0); -- or use LIKE syntax
ALTER TABLE tempdb.student MODIFY id INT(20) NOT NULL AUTO_INCREMENT PRIMARY KEY;
END IF;
If your temporary table needed for the session only, I'd use CREATE TEMPORARY TABLE ... LIKE ... and alter as required.