MySQL之SQL语法及SQL解析顺序源码分析

# 语法顺序
SELECT DISTINCT
	<select_list>
FROM
	<left_table> <join_type>
JOIN <right_table> ON <join_condition>
WHERE
	<where_condition>
GROUP BY
	<group_by_list>
HAVING
	<having_condition>
ORDER BY
	<order_by_condition>
LIMIT <limit_number>

# 解析顺序
FROM <left_table>
ON <join_condition>
<join_type> JOIN <right_table>  -- 这一步和上一步，会循环执行
WHERE <where_condition> -- 这一步会循环执行，多个条件从左往右
GROUP BY <group_by_list>
HAVING <having_condition>
SELECT   -- 分组之后才执行SELECT
DISTINCT <select_list>
ORDER BY <order_by_condition>
LIMIT <limit_number> -- 这一步是MySQL独有的语法，前面都是SQL92标准

select * from seller join product;

select * from seller s join product p on s.id = p.id;

select * from seller s right join product p on s.id = p.id;

-- mysql 8 默认开启了 only_full_group_by
select version(),@@sql_mode;
-- 需要把这个选项关掉，
set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
set session sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
-- 后执行group by 正常
select * from user group by name;

-- 下面无效
select id, distinct name from user;
-- 下面有效
select distinct name from user;