SQL语法
union, union all
UNION 操作符用于合并两个或多个 SELECT 语句的结果集。
注意，UNION 内部的每个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，每个SELECT语句中的列的顺序必须相同。
UNION语法
SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
注释：默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。
UNION ALL语法
SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;
注释：UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。
JOIN
举例我们有两张表，t1和t2。
drop database if exists db1;
create database if not exists db1 character set utf8mb4;
use db1;
drop table if exists t1;
create table if not exists t1(
`id` int not null primary key,
`name` varchar(10) not null);
insert into t1(`id`,`name`)
values(1,"Google"),(2,"Alibaba"),(3,"Oracle"),(4,"PingCAP");
drop table if exists t2;
create table if not exists t2(
`id` int not null primary key,
`address` varchar(10) not null);
insert into t2(`id`,`address`)
values(1,"USA"),(2,"CN"),(5,"UK"),(4,"CN");
select * from t1 order by `id`;
select * from t2 order by `id`;
+----+---------+
| id | name |
+----+---------+
| 1 | Google |
| 2 | Alibaba |
| 3 | Oracle |
| 4 | PingCAP |
+----+---------+
+----+---------+
| id | address |
+----+---------+
| 1 | USA |
| 2 | CN |
| 4 | CN |
| 5 | UK |
+----+---------+
JOIN(INNER JOIN)
内连接是最常见的一种连接，只连接匹配的行。
JOIN语法
SELECT column_name(s) FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name
执行SQL语句
select * from t1 join t2 on t1.id = t2.id order by t1.id;
输出结果如下
+----+---------+----+---------+
| id | name | id | address |
+----+---------+----+---------+
| 1 | Google | 1 | USA |
| 2 | Alibaba | 2 | CN |
| 4 | PingCAP | 4 | CN |
+----+---------+----+---------+
LEFT JOIN(LEFT OUTER JOIN)
LEFT JOIN返回左表的全部行和右表满足ON条件的行，如果左表的行在右表中没有匹配，那么这一行右表中对应数据用NULL代替。
LEFT JOIN 语法
SELECT column_name(s) FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name
LEFT JOIN产生表1的完全集，而2表中匹配的则有值，没有匹配的则以null值取代。
执行SQL语句
select * from t1 left join t2 on t1.id = t2.id order by t1.id;
输出结果如下
+----+---------+------+---------+
| id | name | id | address |
+----+---------+------+---------+
| 1 | Google | 1 | USA |
| 2 | Alibaba | 2 | CN |
| 3 | Oracle | NULL | NULL |
| 4 | PingCAP | 4 | CN |
+----+---------+------+---------+
RIGHT JOIN(RIGHT OUTER JOIN)
RIGHT JOIN返回右表的全部行和左表满足ON条件的行，如果右表的行在左表中没有匹配，那么这一行左表中对应数据用NULL代替。
RIGHT JOIN语法
SELECT column_name(s) FROM table1
RIGHT JOIN table2
ON table1.column_name=table2.column_name
RIGHT JOIN产生表2的完全集，而1表中匹配的则有值，没有匹配的则以null值取代。
执行SQL语句
select * from t1 right join t2 on t1.id = t2.id order by t2.id;
输出结果如下
+------+---------+----+---------+
| id | name | id | address |
+------+---------+----+---------+
| 1 | Google | 1 | USA |
| 2 | Alibaba | 2 | CN |
| 4 | PingCAP | 4 | CN |
| NULL | NULL | 5 | UK |
+------+---------+----+---------+
FULL OUTER JOIN
FULL JOIN 会从左表和右表那里返回所有的行。如果其中一个表的数据行在另一个表中没有匹配的行，那么对面的数据用NULL代替。
FULL OUTER JOIN语法
SELECT column_name(s) FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name
FULL OUTER JOIN产生1和2的并集。但是需要注意的是，对于没有匹配的记录，则会以null做为值。
执行SQL语句
select * from t1 full outer join t2 on t1.id = t2.id order by t1.id;