mysql - mysql 游标中似乎被忽略的地方

Question

我正在尝试使用游标来处理包含字符串的行:

CREATE PROCEDURE REVERT_ALL(IN TABLE_NAME VARCHAR(255))
  BEGIN
    DECLARE bDone INT;
    DECLARE CH_ID INT;
    DECLARE CH_CHANGE CHAR;

    DECLARE curs cursor for 
      SELECT `table_id`, `change_type` FROM mysql_snapshot.db_changes where `table_name` = "rooms";
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET bDone = 1;

    OPEN curs;

    insert into splog set text = concat('SELECT id, `table_name`, table_id, `change_type` FROM mysql_snapshot.db_changes where table_name = ',TABLE_NAME, ';');

    SET bDone = 0;
    REPEAT
      FETCH curs INTO CH_ID, CH_CHANGE;

      insert into splog set text = concat_ws( ' -- ', 'CH_ID ', CH_ID,' TABLE ',TABLE_NAME, ' CH_CHANGE ', CH_CHANGE);

      UNTIL bDone END REPEAT;

      CLOSE curs;
    END;

由于我一天中大部分时间都在寻找的原因，

where `table_name` = "rooms" 

似乎被忽略了。计划将其更改为

where `table_name` = TABLE_NAME 

使用过程参数。我刚刚得到所有行。插入用于记录和调试。

Answer 1

过程参数名称 TABLE_NAME 和表列名称 table_name 之间存在歧义。应该避免它，因为它会导致像这样的晦涩问题。

在本例中，在过程内部，反引号中的 TABLE_NAME、table_name 和 table_name 都是 interpreted as the local variable name (参数名称)。因此，您的条件 'where table_name = ',TABLE_NAME 始终为 true；当您将 TABLE_NAME 替换为 "rooms" 文字时，如果您使用 "rooms" 作为参数调用该过程，则条件始终为 true ，否则始终为假。考虑这个简化的例子:

DROP PROCEDURE IF EXISTS pr;
DROP TABLE IF EXISTS t;
CREATE PROCEDURE pr (IN TABLE_NAME VARCHAR(255)) 
  SELECT `id`, `table_name`, TABLE_NAME FROM t
;

CREATE TABLE t (id INT, `table_name` VARCHAR(255));
INSERT INTO t VALUES (1,'hotels'),(2,'rooms');

CALL pr("rooms");
CALL pr("foo");

你将会得到

MariaDB [test]> CALL pr("rooms");
+------+--------------+------------+
| id   | `table_name` | TABLE_NAME |
+------+--------------+------------+
|    1 | rooms        | rooms      |
|    2 | rooms        | rooms      |
+------+--------------+------------+
2 rows in set (0.00 sec)

MariaDB [test]> CALL pr("foo");
+------+--------------+------------+
| id   | `table_name` | TABLE_NAME |
+------+--------------+------------+
|    1 | foo          | foo        |
|    2 | foo          | foo        |
+------+--------------+------------+
2 rows in set (0.00 sec)

如您所见，无论参数如何，对于所有行，table_name 始终等于 TABLE_NAME。

现在，如果您将 TABLE_NAME 替换为明确的 “rooms”:

DROP PROCEDURE IF EXISTS pr;
DROP TABLE IF EXISTS t;
CREATE PROCEDURE pr (IN TABLE_NAME VARCHAR(255)) 
  SELECT `id`, `table_name`, "rooms" FROM t
;

CREATE TABLE t (id INT, `table_name` VARCHAR(255));
INSERT INTO t VALUES (1,'hotels'),(2,'rooms');

CALL pr("rooms");
CALL pr("foo");

现在，对于所有行的第一次调用，条件 table_name = "rooms" 将为 true，对于所有行的第二次调用，条件为 false:

MariaDB [test]> CALL pr("rooms");
+------+--------------+-------+
| id   | `table_name` | rooms |
+------+--------------+-------+
|    1 | rooms        | rooms |
|    2 | rooms        | rooms |
+------+--------------+-------+
2 rows in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> CALL pr("foo");
+------+--------------+-------+
| id   | `table_name` | rooms |
+------+--------------+-------+
|    1 | foo          | rooms |
|    2 | foo          | rooms |
+------+--------------+-------+
2 rows in set (0.00 sec)

您需要的是参数的不同名称:

DROP PROCEDURE IF EXISTS pr;
DROP TABLE IF EXISTS t;
CREATE PROCEDURE pr (IN T_NAME VARCHAR(255)) 
  SELECT `id`, `table_name`, T_NAME FROM t
;

CREATE TABLE t (id INT, `table_name` VARCHAR(255));
INSERT INTO t VALUES (1,'hotels'),(2,'rooms');

CALL pr("rooms");
CALL pr("foo");

现在，比较table_name = T_NAME实际上是有意义的；

MariaDB [test]> CALL pr("rooms");
+------+------------+--------+
| id   | table_name | T_NAME |
+------+------------+--------+
|    1 | hotels     | rooms  |
|    2 | rooms      | rooms  |
+------+------------+--------+
2 rows in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> CALL pr("foo");
+------+------------+--------+
| id   | table_name | T_NAME |
+------+------------+--------+
|    1 | hotels     | foo    |
|    2 | rooms      | foo    |
+------+------------+--------+
2 rows in set (0.00 sec)