内连接：
mysql> select * from book_wangjing as book_1 inner join user_wangjing as user_1 on book_1.id=user_1.id limit 2;
+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+
| id | book_name | book_author | price | publish_date | id | date1 | date2 | date3 | time |
+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+
| 1 | webdriver 框架实战指南 | 吴老师 | 100 | 2018-05-05 | 1 | 2018-05-05 00:08:24 | 2018-05-05 | 2018-05-05 00:08:24 | 00:08:24 |
| 2 | how google test | 李老师 | 99 | 2018-05-05 | 2 | 2018-05-05 00:08:54 | 2018-05-05 | 2018-05-05 00:08:54 | 00:08:54 |
+----+------------------------+-------------+-------+--------------+----+---------------------+------------+---------------------+----------+
2 rows in set (0.00 sec)
左连接：
mysql> select a.* from author_wangjing as a left join book_wangjing as b on a.author_name=b.book_author;
+----+-------------+--------+
| id | author_name | salary |
+----+-------------+--------+
| 1 | 吴老师 | 10000 |
| 3 | Mr Jackson | 15000 |
| 2 | 张老师 | 13000 |
+----+-------------+--------+
3 rows in set (0.07 sec)
右连接：
mysql> select * from user_wangjing as user_1 right join book_wangjing as book_1 on user_1.id=book_1.id;
+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+
| id | date1 | date2 | date3 | time | id | book_name | book_author | price | publish_date |
+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+
| 1 | 2018-05-05 00:08:24 | 2018-05-05 | 2018-05-05 00:08:24 | 00:08:24 | 1 | webdriver 框架实战指南 | 吴老师 | 100 | 2018-05-05 |
| 2 | 2018-05-05 00:08:54 | 2018-05-05 | 2018-05-05 00:08:54 | 00:08:54 | 2 | how google test | 李老师 | 99 | 2018-05-05 |
| 3 | 2018-05-05 00:08:55 | 2018-05-05 | 2018-05-05 00:08:55 | 00:08:55 | 3 | unit test | 李老师 | 1 | 2018-05-05 |
| 4 | 2018-05-05 00:08:56 | 2018-05-05 | 2018-05-05 00:08:56 | 00:08:56 | 4 | function test | 李老师 | 10 | 2017-11-10 |
| NULL | NULL | NULL | NULL | NULL | 5 | function test | Mr Jackson | 10 | 2013-06-10 |
+------+---------------------+------------+---------------------+----------+----+------------------------+-------------+-------+--------------+
5 rows in set (0.07 sec)
mysql> select a.* from author_wangjing as a right join book_wangjing as b on a.author_name=b.book_author;
+------+-------------+--------+
| id | author_name | salary |
+------+-------------+--------+
| 1 | 吴老师 | 10000 |
| 3 | Mr Jackson | 15000 |
| NULL | NULL | NULL |
| NULL | NULL | NULL |
| NULL | NULL | NULL |
+------+-------------+--------+
union：
mysql> select book_author from book_wangjing union select author_name from author_wangjing;
+-------------+
| book_author |
+-------------+
| 吴老师 |
| 李老师 |
| Mr Jackson |
| 张老师 |
+-------------+
4 rows in set (0.06 sec)
1、 内连接：结果集中出现的a.author_name和b.book_author必须同时在两个表存在
2、 左连接：结果集中出现的a.author_name必须在左表(author_wangjing表)存在且全部显示
3、 右连接：结果集中出现的b.book_author必须在右表(book_wangjing表)存在且全部显示
4、 union：做合并，使用union时左表的数据列要和右表的数据列一样多