求助，下面的存储过程运行的时候有语法错误，求指导
RT，这个存储过程的目的是，用数据库DB1的数据来对比数据库DB2的数据（里面的表结构什么的都一样），把多出来的数据条数找出来，然后给主键（只有一个INT类型的主键）加上一个偏移量，然后把这些数据添加到数据库DB3中（表也是一样的）。

现在编译通过了 但是运行的时候遇到一个错误
“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 'NULL' at line 1”

不解

SQL code

DELIMITER $$DROP PROCEDURE IF EXISTS Pro_Compare_Tab$$CREATE PROCEDURE Pro_Compare_Tab(IN v_DB1 VARCHAR(45), #一号数据库                                 IN v_DB2 VARCHAR(45), #二号数据库                                 IN v_DB3 VARCHAR(45), #三号数据库                                 IN v_Table VARCHAR(45), #3个数据库中都有的表名                                 IN nOffset INT) #主键偏移量BEGINDECLARE v_KEYS  VARCHAR(80);DECLARE v_sql   VARCHAR(1000);DECLARE v_tmp   VARCHAR(100);DECLARE v_tmp2  VARCHAR(100);DECLARE v_SPACE VARCHAR(45);DECLARE v_PKeyExist INT DEFAULT 0;DECLARE n_flag,nNumKeys,n_Index INT DEFAULT 0;DECLARE done INT DEFAULT 0;DECLARE cur_pkey CURSOR FOR SELECT      c.TABLE_NAME,MAX(c.COLUMN_NAME) COLUMN_NAME    FROM      INFORMATION_SCHEMA.TABLE_CONSTRAINTS AS t,      INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS c    WHERE      t.TABLE_NAME = c.TABLE_NAME      AND t.TABLE_SCHEMA = v_DB1      AND t.TABLE_NAME = v_Table      AND t.CONSTRAINT_TYPE = 'PRIMARY KEY'      GROUP BY c.TABLE_NAME      HAVING MAX(c.ORDINAL_POSITION) = 1;DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;  OPEN cur_pkey;  SET n_Index = 1;  LOOP_FLAG:LOOP    FETCH cur_pkey INTO v_SPACE,v_KEYS;    IF done = 1 THEN LEAVE LOOP_FLAG;    END IF;      SET v_PKeyExist = 1;      IF n_Index = 1 THEN        SET @v_tmp = CONCAT('A.',v_KEYS,' = B.',v_KEYS);        SET @v_tmp2 = CONCAT('A.',v_KEYS,' <> B.',v_KEYS);      ELSE        SET @v_tmp = CONCAT(v_tmp,'AND A.',v_KEYS,' = B.',v_KEYS);        SET @v_tmp2 = CONCAT(v_tmp2,' OR A.',v_KEYS,' <> B.',v_KEYS);      END IF;      SET n_Index = n_Index + 1;    END LOOP;  CLOSE cur_pkey;  IF v_PKeyExist <> 0 THEN  SET @v_sql = CONCAT('SELECT 1 into @aa FROM information_schema.TABLES where table_schema = \'',v_DB1,  '\' and table_name = \'',v_Table,'_Back\'');  PREPARE stml FROM @v_sql;  EXECUTE stml;  IF @aa IS NOT NULL THEN    SET @v_sql = CONCAT('TRUNCATE TABLE ',v_DB1,'.',v_Table,'_Back');    PREPARE stml FROM @v_sql;    EXECUTE stml;    SET @v_sql =CONCAT('INSERT INTO ',v_DB1,'.',v_Table,'_Back SELECT * FROM ',v_DB1,'.',v_Table,    ' A LEFT JOIN ',v_DB2,'.',v_Table,' B on ',v_tmp,' WHERE B.',v_KEYS,' IS NULL;');    PREPARE stml FROM @v_sql;    EXECUTE stml;    COMMIT;  ELSE    SET @v_sql =CONCAT( 'CREATE TABLE ',v_DB1,'.',v_Table,'_Back SELECT A.* FROM ',v_DB1,'.',v_Table,    ' A LEFT JOIN ',v_DB2,'.',v_Table,' B ON ',v_tmp,' WHERE B.',v_KEYS,' IS NULL;');    PREPARE stml FROM @v_sql;    EXECUTE stml;    COMMIT;  END IF;  SET @v_sql =CONCAT( 'UPDATE ',v_DB1,'.',v_Table,'_Back A SET ',v_KEYS,' = ',v_KEYS,' + ',V_OFFSET,' WHERE A.',v_KEYS,' < ',V_OFFSET);  PREPARE stml FROM @v_sql;  EXECUTE stml;  COMMIT;  SET @v_sql =CONCAT( 'INSERT INTO ',v_DB3,'.',v_Table,'A  SELECT * FROM ',v_DB1,'.',v_Table,'_Back B WHERE ',v_tmp2);  PREPARE stml FROM @v_sql;  EXECUTE stml;  COMMIT;  SET @v_sql =CONCAT( 'DROP TABLE ',v_DB1,'.',v_Table,'_Back');  PREPARE stml FROM @v_sql;  EXECUTE stml;  COMMIT;  END IF;END$$DELIMITER ;

[解决办法]
在SP中加入
SELECT 变量名
OR
将中间结果插入到临时表中保存，检查结果
[解决办法]
代码中加入
select @v_sql;

看一下实际执行的语句是什么，应该是你生成的SQL语句不正确。