MySQL之prepare⽤法
MySQL官⽅将prepare、execute、deallocate统称为PREPARE STATEMENT，习惯称其为【预处理语句】，下⾯是对其详细的介绍。⽰例代码
PREPARE stmt_name FROM preparable_stmtEXECUTE stmt_name
[USING @var_name [, @var_name] ...] -{DEALLOCATE | DROP} PREPARE stmt_name
举个栗⼦：
mysql> PREPARE pr1 FROM 'SELECT ?+?';Query OK, 0 rows affected (0.01 sec)Statement prepared
mysql> SET @a=1, @b=10 ;
Query OK, 0 rows affected (0.00 sec)mysql> EXECUTE pr1 USING @a, @b;+------+| ?+? |+------+| 11 |+------+
1 row in set (0.00 sec)
mysql> EXECUTE pr1 USING 1, 2; -- 只能使⽤⽤户变量传递。
ERROR 10 (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 '1, 2' at line 1mysql> DEALLOCATE PREPARE pr1;Query OK, 0 rows affected (0.00 sec)
使⽤PAREPARE STATEMENT可以减少每次执⾏SQL的语法分析，⽐如⽤于执⾏带有WHERE条件的SELECT和DELETE，或者UPDATE，或者INSERT，只需要每次修改变量值即可。同样可以防⽌SQL注⼊，参数值可以包含转义符和定界符。适⽤在应⽤程序中，或者SQL脚本中均可。更多⽤法：
同样PREPARE ... FROM可以直接接⽤户变量：
mysql> CREATE TABLE a (a int);Query OK, 0 rows affected (0.26 sec)mysql> INSERT INTO a SELECT 1;Query OK, 1 row affected (0.04 sec)Records: 1 Duplicates: 0 Warnings: 0mysql> INSERT INTO a SELECT 2;Query OK, 1 row affected (0.04 sec)Records: 1 Duplicates: 0 Warnings: 0mysql> INSERT INTO a SELECT 3;Query OK, 1 row affected (0.04 sec)Records: 1 Duplicates: 0 Warnings: 0
mysql> SET @select_test = CONCAT('SELECT * FROM ', @table_name);Query OK, 0 rows affected (0.00 sec)mysql> SET @table_name = 'a';
Query OK, 0 rows affected (0.00 sec)mysql> PREPARE pr2 FROM @select_test;Query OK, 0 rows affected (0.00 sec)Statement preparedmysql> EXECUTE pr2 ;+------+| a |
+------+| 1 || 2 || 3 |+------+
3 rows in set (0.00 sec)
mysql> DROP PREPARE pr2; -- 此处DROP可以替代DEALLOCATEQuery OK, 0 rows affected (0.00 sec)
每⼀次执⾏完EXECUTE时，养成好习惯，须执⾏DEALLOCATE PREPARE … 语句，这样可以释放执⾏中使⽤的所有数据库资源（如游标）。
不仅如此，如果⼀个session的预处理语句过多，可能会达到max_prepared_stmt_count的上限值。预处理语句只能在创建者的会话中可以使⽤，其他会话是⽆法使⽤的。
⽽且在任意⽅式（正常或⾮正常）退出会话时，之前定义好的预处理语句将不复存在。
如果在存储过程中使⽤，如果不在过程中DEALLOCATE掉，在存储过程结束之后，该预处理语句仍然会有效。
