别名：
select abc AS a，def AS d from paramtable where a = ‘test’；
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
排序检索数据：
SELECT name, hp_max FROM heros ORDER BY hp_max DESC
约束返回结果的数量：
SELECT name, hp_max FROM heros ORDER BY hp_max DESC LIMIT 5
SELECT 的执行顺序：
1. 关键词顺序（不能颠倒）
SELECT ... FROM ... WHERE ... GROUP BY ... HAVING ... ORDER BY ...
2. 执行顺序
FROM > WHERE > GROUP BY > HAVING > SELECT的字段 > DISTINCT > ORDER BY > LIMIT
举例：
SELECT DISTINCT player_id, player_name, count(*) as num #顺序5
FROM player JOIN team ON player.team_id = team.team_id #顺序1
WHERE height > 1.80 #顺序2
GROUP BY player.team_id #顺序3
HAVING num > 2 #顺序4
ORDER BY num DESC #顺序6
LIMIT 2 #顺序7
