MySQL 06 LIKE子句、UNION操作符

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1 LIKE

SQL LIKE 子句中使用百分号 %字符来表示任意字符，类似于UNIX或正则表达式中的星号 *。

如果没有使用百分号 %, LIKE 子句与等号 = 的效果是一样的。

mysql> select * from tb1;
+----+-------------+----------+------------+
| id | title       | author   | date       |
+----+-------------+----------+------------+
|  1 | 学习mysql   | lihaogn  | 2018-07-13 |
|  2 | 学习java    | zhangsan | 2018-06-30 |
+----+-------------+----------+------------+
2 rows in set (0.01 sec)

// 使用like进行模糊查询
mysql> select * from tb1 where author like '%li%';
+----+-------------+---------+------------+
| id | title       | author  | date       |
+----+-------------+---------+------------+
|  1 | 学习mysql   | lihaogn | 2018-07-13 |
+----+-------------+---------+------------+
1 row in set (0.00 sec)

2 UNION操作符

MySQL UNION 操作符用于连接两个以上的 SELECT 语句的结果组合到一个结果集合中。多个 SELECT 语句会删除重复的数据。

数据表：

mysql> select * from websites;
+----+----------+---------------------------+---------+-------+
| id | webName  | url                       | country | alexa |
+----+----------+---------------------------+---------+-------+
|  1 | google   | https://www.google.cm/    | USA     |     1 |
|  2 | 淘宝     | https://www.taobao.com/   | CN      |    13 |
|  3 | 微博     | http://weibo.com/         | CN      |    20 |
|  4 | Facebook | https://www.facebook.com/ | USA     |     3 |
+----+----------+---------------------------+---------+-------+
4 rows in set (0.00 sec)

mysql> select * from apps;
+----+------------+-------------------------+---------+
| id | app_name   | url                     | country |
+----+------------+-------------------------+---------+
|  1 | QQ APP     | http://im.qq.com/       | CN      |
|  2 | 微博 APP   | http://weibo.com/       | CN      |
|  3 | 淘宝 APP   | https://www.taobao.com/ | CN      |
+----+------------+-------------------------+---------+
3 rows in set (0.00 sec)

操作：

// 不会显示重复的值
mysql> select country from websites
    -> union
    -> select country from apps
    -> order by country;
+---------+
| country |
+---------+
| CN      |
| USA     |
+---------+
2 rows in set (0.00 sec)

// 加上all，显示全部的值
mysql> select country from websites
    -> union all
    -> select country from apps
    -> order by country;
+---------+
| country |
+---------+
| CN      |
| CN      |
| CN      |
| CN      |
| CN      |
| USA     |
| USA     |
+---------+
7 rows in set (0.00 sec)

mysql> select country,webName from websites
    -> where country='CN'
    -> union all
    -> select country,app_name from apps
    -> where country='CN'
    -> order by country;
+---------+------------+
| country | webName    |
+---------+------------+
| CN      | 淘宝       |
| CN      | 微博 APP   |
| CN      | 微博       |
| CN      | 淘宝 APP   |
| CN      | QQ APP     |
+---------+------------+
5 rows in set (0.00 sec)