MySQL SQL预处理(Prepared)的语法实例与注意事项

# 定义预处理语句
PREPARE stmt_name FROM preparable_stmt;
# 执行预处理语句
EXECUTE stmt_name [USING @var_name [, @var_name] ...];
# 删除(释放)定义
{DEALLOCATE | DROP} PREPARE stmt_name;

mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> SET @a = 3;
Query OK, 0 rows affected (0.00 sec)

mysql> SET @b = 4;                                                   
Query OK, 0 rows affected (0.00 sec)

mysql> EXECUTE stmt1 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|          5 |
+------------+
1 row in set (0.00 sec)

mysql> DEALLOCATE PREPARE stmt1;                                     
Query OK, 0 rows affected (0.00 sec)

mysql> SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
Query OK, 0 rows affected (0.00 sec)

mysql> PREPARE stmt2 FROM @s;
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> SET @c = 6;
Query OK, 0 rows affected (0.00 sec)

mysql> SET @d = 8;
Query OK, 0 rows affected (0.00 sec)

mysql> EXECUTE stmt2 USING @c, @d;
+------------+
| hypotenuse |
+------------+
|         10 |
+------------+
1 row in set (0.00 sec)

mysql> DEALLOCATE PREPARE stmt2;
Query OK, 0 rows affected (0.00 sec)

mysql> SET @skip = 100; SET @numrows = 3;
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM t1 LIMIT @skip, @numrows;
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 '@skip, @numrows' at line 1

mysql> PREPARE stmt3 FROM "SELECT * FROM t1 LIMIT ?, ?";
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> EXECUTE stmt3 USING @skip, @numrows;
+-----+--------+
| a   | filler |
+-----+--------+
| 100 | filler |
| 101 | filler |
| 102 | filler |
+-----+--------+
3 rows in set (0.00 sec)

mysql> DEALLOCATE PREPARE stmt3;
Query OK, 0 rows affected (0.00 sec)

mysql> SET @table = 't2';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @s = CONCAT('SELECT * FROM ', @table);
Query OK, 0 rows affected (0.00 sec)

mysql> PREPARE stmt4 FROM @s;
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> EXECUTE stmt4;
+------+-------+-------+
| id   | score | grade |
+------+-------+-------+
|    1 |    99 | A     |
|    2 |    81 | B     |
|    3 |    55 | D     |
|    4 |    69 | C     |
+------+-------+-------+
4 rows in set (0.00 sec)

mysql> DROP PREPARE stmt4;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'max_prepared%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| max_prepared_stmt_count | 16382 |
+-------------------------+-------+
1 row in set (0.00 sec)