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MySQL学习 (三) Limit-Distinct-Union

Limit

简介

• 限制获得的记录数量

mysql> SELECT * FROM teacher_class limit 1;
+----+--------+--------+-----------+------+------+------------+------------+
| id | t_name | gender | c_name    | room | days | begin_date | end_date   |
+----+--------+--------+-----------+------+------+------------+------------+
|  1 | Gene   | male   | python001 | 334  |   22 | 2013-03-31 | 2013-05-05 |
+----+--------+--------+-----------+------+------+------------+------------+
1 row in set (0.00 sec)

语法

Limit offset, row_count

# offset 偏移量，从0开始;
# row_count，总记录数，长度，不是开始和结束, 如果数量大于余下的记录数，则获取所有余下的记录数，默认值0

mysql> SELECT * FROM teacher_class limit 3,4;
+----+---------+--------+-----------+------+------+------------+------------+
| id | t_name  | gender | c_name    | room | days | begin_date | end_date   |
+----+---------+--------+-----------+------+------+------------+------------+
|  4 | Emma    | female | python004 | 338  |   20 | 2013-03-15 | 2013-04-05 |
|  5 | Janice  | female | python007 | 332  |   27 | 2013-04-15 | 2013-05-10 |
|  6 | Jessica | female | python008 | 332  |   28 | 2013-05-15 | 2013-06-08 |
+----+---------+--------+-----------+------+------+------------+------------+
3 rows in set (0.00 sec)

Distinct

简介

• distinct: 去掉重复记录， 指的是字段值，都相同的记录，而不是部分字段相同的记录
• All: 表示所有，默认就是all的行为

mysql> SELECT days FROM teacher_class;
+------+
| days |
+------+
|   22 |
|   22 |
|   55 |
|   20 |
|   27 |
|   28 |
+------+
6 rows in set (0.00 sec)

mysql> SELECT DISTINCT days FROM teacher_class;
+------+
| days |
+------+
|   22 |
|   55 |
|   20 |
|   27 |
|   28 |
+------+
5 rows in set (0.00 sec)

mysql> SELECT days, begin_date FROM teacher_class;
+------+------------+
| days | begin_date |
+------+------------+
|   22 | 2013-03-31 |
|   22 | 2013-05-31 |
|   55 | 2013-02-15 |
|   20 | 2013-03-15 |
|   27 | 2013-04-15 |
|   28 | 2013-05-15 |
+------+------------+
6 rows in set (0.01 sec)

mysql> SELECT DISTINCT days, begin_date FROM teacher_class;
+------+------------+
| days | begin_date |
+------+------------+
|   22 | 2013-03-31 |
|   22 | 2013-05-31 |
|   55 | 2013-02-15 |
|   20 | 2013-03-15 |
|   27 | 2013-04-15 |
|   28 | 2013-05-15 |
+------+------------+
6 rows in set (0.00 sec)

union联合查询

简介

• 将多条select语句的结果，合并到一起，称为联合

• 使用union关键字，联合两个select语句即可

• 注意 如果union查询结果有重复，会自动去掉重复的数据；如果想得到全部数据，通过UNION ALL来显示数据

• 多个select语句检索时，字段要一致; 数据类型，尽量保持一致；如果不一致，应该能让MySQL能够做类型转换

• 检索结果列名称的问题：根据第一条select语句的字段，进行名称排列

• 场景

  1. 获得数据的条件，出现逻辑冲突，或者很难在一个逻辑内表示，不好拆分，分成多个逻辑，分别实现，然后合并到一起

mysql> SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ORDER BY days DESC LIMIT 1;
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   55 |
+---------+------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ORDER BY days DESC LIMIT 1;
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   28 |
+---------+------+
1 row in set (0.00 sec)

# 联合查询语法

mysql>(SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ORDER BY days DESC LIMIT 1) UNION (SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ORDER BY days DESC LIMIT 1);
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   55 |
| Jessica |   28 |
+---------+------+
2 rows in set (0.00 sec)

子语句的结果排序

1. 将子语句包裹在括号里
2. 子语句的order by需要使用limit才能生效，否则，排序失效

所有结果进行排序

1. 只需要在最后一个select子语句后，使用ORDER BY进行排序

mysql> (SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ) \
    -> UNION (SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ) ORDER BY days;
+---------+------+
| t_name  | days |
+---------+------+
| Mona    |   22 |
| Emma    |   28 |
| Jessica |   28 |
| Mona    |   55 |
| Jessica |   55 |
+---------+------+
5 rows in set (0.00 sec)