LeetCode SQL刷题（3）- 表连接问题

目录

1. UNION和UNION ALL

2. Join（四种连接方式）

1. UNION和UNION ALL

要求：

UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同

区别1 - Union会进行去重：

a. Union对两个结果集进行并集操作, 不包括重复行,相当于distinct, 同时进行默认规则的排序
b. Union all对两个结果集进行并集操作, 包括重复行, 即所有的结果全部显示, 不管是不是重复

区别2 - Union会进行排序：

a. Union默认排序，如果第一个字段重复，则按第二个字段排序，以此类推
b. Union all不排序

共性：

a. Union/Union all的前后两张表都不需要加括号

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

b. Union后可以直接使用两张表的新建列名做排序

SELECT 'succeeded' AS period_state,
MIN(success_date) AS start_date,
MAX(success_date) AS end_date
FROM
(SELECT *, 
ROW_NUMBER() OVER(ORDER BY success_date ASC) AS rk,
DATE_SUB(success_date, INTERVAL ROW_NUMBER() OVER(ORDER BY success_date ASC) DAY) AS diff
FROM Succeeded
WHERE success_date BETWEEN '2019-01-01' AND '2019-12-31') a
GROUP BY diff
UNION ALL
SELECT 'failed' AS period_state,
MIN(fail_date) AS start_date,
MAX(fail_date) AS end_date
FROM
(SELECT *, 
ROW_NUMBER() OVER(ORDER BY fail_date ASC) AS rk,
DATE_SUB(fail_date, INTERVAL ROW_NUMBER() OVER(ORDER BY fail_date ASC) DAY) AS diff
FROM Failed
WHERE fail_date BETWEEN '2019-01-01' AND '2019-12-31') a
GROUP BY diff
ORDER BY start_date

2. Join（四种连接方式）

1. 内连接：Inner Join
2. 外连接：Left Outer Join，Right Outer Join（Outer可省略）
3. 全连接：Full Outer Join（Outer可省略）-- 只要其中某个表存在匹配，Full Join 关键字就会返回行（左连接＋右连接）
4. 笛卡尔积连接：Cross Join -- 两表直接笛卡尔积的结果数量是两表的数据量相乘（列相加，行相乘）

注意：Mysql中没有Full Join，可以左连接和右连接进行Union

SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.name = table2.name

UNION

SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.name =table2.name

注意：笛卡尔积连接有两种写法

select * from a  CROSS JOIN  b;

或者

select * from a, b