5、如果是out join(left),还需要把没匹配上的行数添加和join的数据合并,生成一张临时表 6、where,对临时表进行过滤 7、 别人好的解释:(1.)select语句的执行顺序 Processing Order of the SELECT statement The following steps show the processing order for a SELECT statement. 1.FROM 2.ON 3.JOIN 4.WHE...
SELECT e.id, e.name, d.dept_name:选择员工的id、name和部门的dept_name。 FROM employees e:从employees表中选择,并给它一个别名e。 LEFT JOIN departments d:将departments表使用 LEFT JOIN 连接到employees表,并给它一个别名d。 ON e.dept_id = d.id:定义连接条件,即员工的dept_id应与部门的id匹配。
mysql> select * from t1, t2; 2、左连接 两表关联,左表全部保留,右表关联不上用null表示。 mysql> select * from t1 left join t2 on t1.id = t2.id; 3、右连接 右表全部保留,左表关联不上的用null表示。 mysql> select * from t1 right join t2 on t1.id =t2.id; 4、内连接 两表关联,...
left JOIN edu_score score ON = score.stu_id and stu.stu_name='盲僧'; -- # 区别于 -- # 验证结论5:主表的筛选条件在 where 后 SELECT * from edu_student stu left JOIN edu_score score ON = score.stu_id where stu.stu_name='盲僧'; 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11....
select c.* from hotel_info_original c left join hotel_info_collection h on c.hotel_type=h.hotel_type and c.hotel_id =h.hotel_id where h.hotel_id is null 这个sql是用来查询出c表中有h表中无的记录,所以想到了用left join的特性(返回左边全部记录,右表不满足匹配条件的记录对应行返回null)来...
mysql> SELECT -> r.id, -> r.role_name AS role, -> count( u.sex ) AS sex -> FROM -> role r -> LEFT JOIN USER u ON r.id = u.role_id -> WHERE -> u.sex = 2 -> GROUP BY -> r.role_name -> ORDER BY -> r.id ASC; +---+---+---+ | id | role | sex | ...
原因:LEFT JOIN 左表满,右表不存在数据时,也会显示左边的数据;当右表不存在时,条件在前面也无法限制住b.currency_code = a.currency_code 错误2、-- 语句2、显示1101条数据 SELECT a.* FROM td_neo_crm_order_pay_detail AS a LEFT JOIN (SELECT * FROM td_neo_crm_order_pay_detail WHERE pay_type...
SELECT m.*, EXISTS ( SELECT 1 FROM my_db_name.mst_user_status s WHERE s.mst_level_id = m.id AND s.mst_user_id = '363fdeea-5330-490a-b4db-34e32a3526d6' ) AS checked FROM my_db_name.mst_level m; 或者使用LEFT JOIN,其中ON子句中设置了条件: SELECT m.*, s.mst_level_id...
select * from a left join b on a2=b2 UNION select * from a right join b on a2=b2; #结果如图9 oracle 数据库有直接语法: select * from a full join b on a2=b2; 7.非等值连接 非等值连接:多个表直接没有连接条件,只有过滤条件的连接。(这里可先对各个表的行过滤,过滤后在进行交叉连接,也可...