mysql的leftjion改写notin或notexists

需求：查询数据库gip_demo比数据库epmp多的表和字段：

1.优先考虑使用not exist：

SELECT
			a.table_name,
			a.column_name
		FROM
			information_schema.COLUMNS a
		WHERE
			a.TABLE_NAME REGEXP '^s_'
		AND a.TABLE_SCHEMA = 'gip_demo'
and 
	NOT EXISTS (
		SELECT
			1
		FROM
			information_schema.COLUMNS b
		WHERE
			TABLE_NAME REGEXP '^s_'
		AND TABLE_SCHEMA = 'epmp'
		AND a.column_name = b.column_name
		AND a.table_name = b.table_name
	);

查询时间：54.67s

2.使用not in:

select a.table_name,a.column_name from 
(SELECT
	i.table_name,i.column_name
FROM
	information_schema.COLUMNS i
WHERE
	i.TABLE_NAME REGEXP '^s_' 
AND i.TABLE_SCHEMA = 'gip_demo') a 
where (a.table_name,a.column_name)
not in(	SELECT
		b.table_name,b.column_name
	FROM
		information_schema.COLUMNS b
	WHERE
		TABLE_NAME REGEXP '^s_'
AND TABLE_SCHEMA = 'epmp');

执行时间：40.341s

3.使用left join:

SELECT
     a.table_name na,a.column_name
FROM
	(
		SELECT
			a.table_name,
			a.column_name
		FROM
			information_schema. COLUMNS a
		WHERE
			a.TABLE_NAME REGEXP '^s_'
		AND a.TABLE_SCHEMA = 'gip_demo'
	) a
left JOIN (
	SELECT
		b.table_name,
		b.column_name
	FROM
		information_schema. COLUMNS b
	WHERE
		b.TABLE_NAME REGEXP '^s_'
	AND b.TABLE_SCHEMA = 'epmp'
) b ON a.table_name = b.table_name  AND a.column_name = b.column_name 
WHERE
	b.table_name IS NULL;

执行时间：0.730s

总结：sql语句并没有铁律说用什么方式实现就一定会快，一定要看使用场景。