检查数据表中某个变量字段是否存在。
DELIMITER $$  
DROP PROCEDURE IF EXISTS CHECK_VAR$$  

CREATE DEFINER='root'@'localhost' PROCEDURE CHECK_VAR(IN tablename VARCHAR(50),IN variable VARCHAR(50))

BEGIN
SET @SQL=CONCAT('IF EXISTS (SELECT * FROM information_schema.columns where table_name=\'',tablename,'\' and column_name=\'',variable,'\') THEN RETURN "YES" ELSE RETURN "NO"');
PREPARE stmt FROM @SQL;  
  EXECUTE stmt;  
  DEALLOCATE PREPARE stmt;  
END$$

DELIMITER ;

CALL CHECK_VAR('fruits','f_id')出错



------解决方案--------------------
create PROCEDURE CHECK_VAR(tablename VARCHAR(50),variable VARCHAR(50))
BEGIN
if EXISTS (SELECT * FROM information_schema.columns where table_name=tablename and column_name=variable) 
then
select "YES"; 
else
 select "NO";
end if;
end$



mysql> call CHECK_VAR('test','a');
-> $
+----+
| NO |
+----+
| NO |
+----+
1 row in set (0.00 sec)
------解决方案--------------------
(SELECT * FROM information_schema.columns where table_name=tablename and column_name=variable)
显然，红色部分仅做一个WHERE中的变量，而不是表名和变量名。