mysql的json查询：
1、一条普通的SQL
SELECT <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>
2、Mysql 解析读取后的SQL
FROM <left_table>
ON <join_condition> <join_type> JOIN <right_table>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
SELECT DISTINCT <select_list>
ORDER BY <order_by_condition>
LIMIT <limit_number>
mysql解析SQL的时候先从FROM 开始，应为他需要先知道数据来源那些表。
笛卡尔积：A表10条 B表5条 ，笛卡尔积=10*5。