## B.3.6 Table Definition-Related Issues 

## B.3.6.1 Problems with ALTER TABLE 

If you get a duplicate-key error when using ALTER TABLE  to change the character set or collation of a character column, the cause is either that the new column collation maps two keys to the same value or that the table is corrupted. In the latter case, you should run REPAIR TABLE  on the table. REPAIR TABLE works for MyISAM , ARCHIVE , and CSV  tables. 

If you use ALTER TABLE  on a transactional table or if you are using Windows, ALTER TABLE  unlocks the table if you had done a LOCK TABLE  on it. This is done because InnoDB  and these operating systems cannot drop a table that is in use. 

## B.3.6.2 TEMPORARY Table Problems 

Temporary tables created with CREATE TEMPORARY TABLE  have the following limitations: 

• TEMPORARY  tables are supported only by the InnoDB , MEMORY , MyISAM , and MERGE  storage engines. 

•Temporary tables are not supported for NDB Cluster. 

•The SHOW TABLES  statement does not list TEMPORARY  tables. 

•To rename TEMPORARY  tables, RENAME TABLE  does not work. Use ALTER TABLE  instead: 

ALTER TABLE old_name RENAME new_name; 

•You cannot refer to a TEMPORARY  table more than once in the same query. For example, the following does not work: 

SELECT * FROM temp_table JOIN temp_table AS t2; 

The statement produces this error: 

ERROR 1137: Can’t reopen table: ’temp_table’ 

You can work around this issue if your query permits use of a common table expression (CTE) rather than a TEMPORARY  table. For example, this fails with the Can’t reopen table  error: 

CREATE TEMPORARY TABLE t SELECT 1 AS col_a, 2 AS col_b; SELECT * FROM t AS t1 JOIN t AS t2; 

To avoid the error, use a WITH  clause that defines a CTE, rather than the TEMPORARY  table: 

WITH cte AS (SELECT 1 AS col_a, 2 AS col_b) SELECT * FROM cte AS t1 JOIN cte AS t2; 

•The Can’t reopen table  error also occurs if you refer to a temporary table multiple times in a stored function under different aliases, even if the references occur in different statements within the function. It may occur for temporary tables created outside stored functions and referred to across multiple calling and callee functions. 

•If a TEMPORARY  is created with the same name as an existing non- TEMPORARY  table, the non- TEMPORARY  table is hidden until the TEMPORARY  table is dropped, even if the tables use different storage engines. 

•There are known issues in using temporary tables with replication. See Section 17.5.1.31, “Replication and Temporary Tables” , for more information. 

