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MySQL自连接与子查询方式

作者:_周游

这篇文章主要介绍了MySQL自连接与子查询方式,具有很好的参考价值,希望对大家有所帮助,如有错误或未考虑完全的地方,望不吝赐教

1. 自连接

自连接是表自身与自身做笛卡尔积,在SQL中进行条件查询,都是指定某一列或多个列之间进行关系运算,无法进行行与行之间的运算,在某些情况下需要对行与行之间进行关系运算,就要使用到自连接。

自连接的本质是将行转为列

示例:显示所有“课程id为3”比“课程id为1”成绩高的成绩信息:

(成绩信息在score表中)

(1)对score进行自连接(别名求笛卡尔积)并删除无效信息:

mysql> select* from score as s1, score as s2 where s1.student_id = s2.student_id;

(2)选出第一列id=1的课程与第二列id=3的课程:

mysql> select* from score as s1, score as s2
    -> where s1.student_id = s2.student_id
    -> and s1.course_id = 1
    -> and s2.course_id = 3;

(该结果表示有三个同学同时选修了这两门课程)

(3)增加左列成绩小于右列成绩条件,SQL指令与查询结果为:

mysql> select* from score as s1,score as s2
    -> where s1.student_id = s2.student_id
    -> and s1.course_id = 1
    -> and s2.course_id = 3
    -> and s1.score < s2.score;
+-------+------------+-----------+-------+------------+-----------+
| score | student_id | course_id | score | student_id | course_id |
+-------+------------+-----------+-------+------------+-----------+
|  70.5 |          1 |         1 |  98.5 |          1 |         3 |
|  33.0 |          3 |         1 |  68.0 |          3 |         3 |
+-------+------------+-----------+-------+------------+-----------+
2 rows in set (0.00 sec)

注:

(1)不能直接进行自连接:

mysql> select* from score,score;
ERROR 1066 (42000): Not unique table/alias: 'score'

需要为表指定两个别名,即:

mysql> select* from score as s1, score as s2;

2. 子查询(嵌套查询)

子查询是指嵌入其他SQL语句中的select语句,即将多个查询语句合并为一个语句;

2.1 子查询分类

(1)单行子查询:查询结果只有一条记录;

(2)多行子查询:查询结果为多条记录;

2.2 单行子查询示例1:查询不想毕业同学的同班同学

(1)分步查询SQL指令及查询结果为:

mysql> select classes_id from student where name="不想毕业";
+------------+
| classes_id |
+------------+
|          1 |
+------------+
1 row in set (0.00 sec)

mysql> select name from student where classes_id =1;
+------------+
| name       |
+------------+
| 黑旋风李逵 |
| 菩提老祖   |
| 白素贞     |
| 许仙       |
| 不想毕业   |
+------------+
5 rows in set (0.00 sec)

(2)子查询SQL指令及查询结果为:

mysql> select name from student where classes_id = (select classes_id from student where name="不想毕业");
+------------+
| name       |
+------------+
| 黑旋风李逵 |
| 菩提老祖   |
| 白素贞     |
| 许仙       |
| 不想毕业   |
+------------+
5 rows in set (0.00 sec)

即将条件查询的某一个值替换为一个select查询语句;

2.3 多行子查询示例2:查询语文或英语课程的信息成绩

先查询出两个课程的课程id,再根据course_id在score表中查询;

(1)分步查询SQL指令及查询结果为:

mysql> select id from course where name="语文" or name="英文";
+----+
| id |
+----+
|  4 |
|  6 |
+----+
2 rows in set (0.00 sec)

mysql> select* from score where course_id in(4,6);
+-------+------------+-----------+
| score | student_id | course_id |
+-------+------------+-----------+
|  98.0 |          1 |         6 |
|  72.0 |          4 |         6 |
|  43.0 |          6 |         4 |
|  79.0 |          6 |         6 |
|  92.0 |          7 |         6 |
+-------+------------+-----------+
5 rows in set (0.00 sec)

(2)子查询SQL指令及查询结果为:

mysql> select* from score where course_id in(select id from course where name="语文" or name="英文");
+-------+------------+-----------+
| score | student_id | course_id |
+-------+------------+-----------+
|  98.0 |          1 |         6 |
|  72.0 |          4 |         6 |
|  43.0 |          6 |         4 |
|  79.0 |          6 |         6 |
|  92.0 |          7 |         6 |
+-------+------------+-----------+
5 rows in set (0.00 sec)

3. 合并查询

合并查询就是将两个查询语句的结果合并到一起;

3.1 示例1:查询id=3或者名字为英文的课程

(1)使用逻辑或实现查询:

mysql> select* from course where id<3 or name="英文";
+----+--------------+
| id | name         |
+----+--------------+
|  1 | Java         |
|  2 | 中国传统文化 |
|  6 | 英文         |
+----+--------------+
3 rows in set (0.00 sec)

(2)使用union关键字进行合并查询:

mysql> select* from course where id<3 union select* from course where name="英文";
+----+--------------+
| id | name         |
+----+--------------+
|  1 | Java         |
|  2 | 中国传统文化 |
|  6 | 英文         |
+----+--------------+
3 rows in set (0.00 sec)

注:

(1)union与逻辑或的区别:

逻辑或只能对一张表的查询结果进行合并,但union可以对多张表的查询结果进行合并(要求多个结果的列须对应);

(2)union与union all的区别:

使用union关键字对多个查询结果进行合并时会自动去重,但unionall不会去重;

总结

以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。

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