select基本查询语法

作用	sql例句
查询对多个列	SELECT name, hp_max, mp_max, attack_max, defense_max FROM heros
起别名	SELECT name AS n, hp_max AS hm, mp_max AS mm, attack_max AS am, defense_max AS dm FROM heros
查询常数	SELECT '王者荣耀' as platform, name FROM heros
去除重复行	SELECT DISTINCT attack_range FROM heros
排序ASC，DESC	SELECT name, hp_max FROM heros ORDER BY mp_max, hp_max DESC
返回结果数量，提升效率	SELECT name, hp_max FROM heros ORDER BY hp_max DESC LIMIT 5

select语句sql关键字顺序

SELECT ... FROM ... WHERE ... GROUP BY ... HAVING ... ORDER BY ...LIMIT ...

sql语句实际执行顺序

FROM > WHERE > GROUP BY > HAVING > SELECT的字段 > DISTINCT > ORDER BY > LIMIT

提升查询效率的方法

• 限定返回结果的数量 limit
• 设置过滤条件 where having
• 避免全表扫描