SQL查询语句有多种,下面总结下。首先先建三张表用于后面的实验
-- 学生表,记录学生信息
CREATE TABLE student(
sno VARCHAR(10),
sname VARCHAR(10),
ssex ENUM('男','女'),
sage INT,
sdept VARCHAR(10),
PRIMARY KEY(sno)
);
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
| 201215123 | 王敏 | 女 | 18 | MA |
| 201215125 | 张立 | 男 | 19 | IS |
+-----------+-------+------+------+-------+
-- 课程表,记录课程信息,cpno是指当前记录的先行课程的cno
CREATE TABLE course(
cno INT AUTO_INCREMENT,
cname VARCHAR(10),
cpno INT,
ccredit INT NOT NULL,
PRIMARY KEY(cno),
FOREIGN KEY(cpno) REFERENCES course(cno)
);
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
| 201215123 | 王敏 | 女 | 18 | MA |
| 201215125 | 张立 | 男 | 19 | IS |
+-----------+-------+------+------+-------+
-- 选课记录表,记录选课信息
CREATE TABLE sc (
sno VARCHAR(10),
cno INT,
grade INT
);
+-----------+------+-------+
| sno | cno | grade |
+-----------+------+-------+
| 201215121 | 1 | 92 |
| 201215121 | 2 | 85 |
| 201215121 | 3 | 88 |
| 201215122 | 1 | 90 |
| 201215122 | 2 | 80 |
+-----------+------+-------+
仅涉及一张表的查询语句称为单表查询语句,举个栗子。
SELECT * FROM student;
SELECT FROM student WHERE sage>=20;
这些语句仅涉及了一张表,所以是单表查询语句。
与单标查询对应,涉及多个表的查询为多表查询,其中又分为连接查询、嵌套查询、 派生表查询、集合查询。
连接查询是数据库查询中最常用的一种查询语句,是指通过连接字段和连接条件连接多个表从而进行查询,连接查询又分为小类:等值连接、非等值连接 、自然连接、外连接、内连接、自身连接。
当连接条件是等于号(=)时的连接称之为等值连接,相反,当连接条件不是等于号就是非等值连接。
-- 查询每个学生的选修课情况,连接条件是等于,连接字段是sno
SELECT * FROM student,sc WHERE student.sno = sc.sno;
+-----------+-------+------+------+-------+-----------+------+-------+
| sno | sname | ssex | sage | sdept | sno | cno | grade |
+-----------+-------+------+------+-------+-----------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 80 |
+-----------+-------+------+------+-------+-----------+------+-------+
该连接操作的过程是,首先拿出student表中的第一条记录,然后根据连接条件和连接字段,与 sc表中的所有记录进行匹配,合适接连接起来形成结果表中的一个元组。然后再拿student表的 第二条记录与sc表进行匹配,第三条记录...,如此反复直到取完。这一匹配算法称为嵌套循环连接算法
内连接就是等值连接或者非等值连接的另一种写法,写法有INNER JOIN ON或者CROSS JOIN USING两种
-- 使用内连接查询每个学生的选修课情况,查询结果和使用上面的等值连接一样。
-- 在MySQL中,INNER可省略,CROSS JOIN= INNER JOIN = INNER
SELECT * FROM student INNER JOIN sc ON student.sno=sc.sno;
SELECT * FROM student JOIN sc ON student.sno=sc.sno;
SELECT * FROM student CROSS JOIN sc USING(sno);
+-----------+-------+------+------+-------+-----------+------+-------+
| sno | sname | ssex | sage | sdept | sno | cno | grade |
+-----------+-------+------+------+-------+-----------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 80 |
+-----------+-------+------+------+-------+-----------+------+-------+
外连接的存在可以弥补内连接仅匹配符合条件的元组的缺陷,也就是说,内连接仅能查询出两个表中符合连接条件的元组 ,而外连接可以在某种程度上弥补这种缺陷。外连接分为左外连接(以JOIN关键字左边的表为基准,没有匹配的记录则置NULL),右外连接(以JOIN关键字右边的表为基准) ,全外连接(以JOIN关键字左右两边的表为基准)。其中MySQL不支持全外连接,但是可以用集合查询做到,即将左外连接的查询结果和右外连接的查询结果做UNION ALL操作。
-- 左外连接,以左边的表student为基准。
在MySQL中,OUTER关键字在MySQL中可省略 LEFT JOIN=LEFT OUTER JOIN,RIGHT JOIN=RIGHT OUTER JOIN
SELECT * FROM student LEFT OUTER JOIN sc ON student.sno=sc.sno;
SELECT * FROM student LEFT JOIN sc ON student.sno=sc.sno;
+-----------+-------+------+------+-------+-----------+------+-------+
| sno | sname | ssex | sage | sdept | sno | cno | grade |
+-----------+-------+------+------+-------+-----------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 80 |
| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL |
| 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL |
+-----------+-------+------+------+-------+-----------+------+-------+
-- 右外连接,注意sc和student换了位置
SELECT * FROM sc RIGHT OUTER JOIN student ON student.sno=sc.sno;
+-----------+------+-------+-----------+-------+------+------+-------+
| sno | cno | grade | sno | sname | ssex | sage | sdept |
+-----------+------+-------+-----------+-------+------+------+-------+
| 201215121 | 1 | 92 | 201215121 | 李勇 | 男 | 20 | CS |
| 201215121 | 2 | 85 | 201215121 | 李勇 | 男 | 20 | CS |
| 201215121 | 3 | 88 | 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 1 | 90 | 201215122 | 刘晨 | 女 | 19 | CS |
| 201215122 | 2 | 80 | 201215122 | 刘晨 | 女 | 19 | CS |
| NULL | NULL | NULL | 201215123 | 王敏 | 女 | 18 | MA |
| NULL | NULL | NULL | 201215125 | 张立 | 男 | 19 | IS |
+-----------+------+-------+-----------+-------+------+------+-------+
-- 全外连接
SELECT * FROM sc FULL JOIN student ON student.sno=sc.sno;
ERROR 1054 (42S22): Unknown column 'sc.sno' in 'on clause'
-- 注意是UNION ALL,而非UNION,UNION有个去重效果
SELECT * FROM student LEFT OUTER JOIN sc ON student.sno=sc.sno
UNION ALL
SELECT * FROM student RIGHT OUTER JOIN sc ON student.sno=sc.sno;
+-----------+-------+------+------+-------+-----------+------+-------+
| sno | sname | ssex | sage | sdept | sno | cno | grade |
+-----------+-------+------+------+-------+-----------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 80 |
| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL | NULL |
| 201215125 | 张立 | 男 | 19 | IS | NULL | NULL | NULL |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 | 2 | 80 |
+-----------+-------+------+------+-------+-----------+------+-------+
在等值连接中去除相同的属性即为自然连接或称全自然连接,左自然连接以左表为基准匹配, 右自然连接以右表为基准匹配
-- 查询每个学生的选修课情况,自然连接,去除相同的属性sno
SELECT student.sno,student.sname,student.ssex,student.sage,student.sdept,sc.cno,sc.grade
FROM student,sc WHERE student.sno = sc.sno;
SELECT * FROM student NATURAL JOIN sc;
+-----------+-------+------+------+-------+------+-------+
| sno | sname | ssex | sage | sdept | cno | grade |
+-----------+-------+------+------+-------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 2 | 80 |
+-----------+-------+------+------+-------+------+-------+
SELECT * FROM student NATURAL LEFT JOIN sc;
+-----------+-------+------+------+-------+------+-------+
| sno | sname | ssex | sage | sdept | cno | grade |
+-----------+-------+------+------+-------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 2 | 80 |
| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL |
| 201215125 | 张立 | 男 | 19 | IS | NULL | NULL |
+-----------+-------+------+------+-------+------+-------+
-- sc和student位置交换了,仍已student为基准,以为王敏、张立没有选课,所以有NULL字段
SELECT * FROM sc NATURAL RIGHT JOIN student;
+-----------+-------+------+------+-------+------+-------+
| sno | sname | ssex | sage | sdept | cno | grade |
+-----------+-------+------+------+-------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS | 1 | 92 |
| 201215121 | 李勇 | 男 | 20 | CS | 2 | 85 |
| 201215121 | 李勇 | 男 | 20 | CS | 3 | 88 |
| 201215122 | 刘晨 | 女 | 19 | CS | 1 | 90 |
| 201215122 | 刘晨 | 女 | 19 | CS | 2 | 80 |
| 201215123 | 王敏 | 女 | 18 | MA | NULL | NULL |
| 201215125 | 张立 | 男 | 19 | IS | NULL | NULL |
+-----------+-------+------+------+-------+------+-------+
顾名思义,自身连接就是一张表,自己和自己连接。
-- '数据库'的先修课信息,连接条件是course1.cno = course2.cpno
SELECT * FROM course AS course1,course AS course2
WHERE course1.cno = course2.cpno
AND course1.cno = 4
+-----+--------+------+---------+-----+--------+------+---------+
| cno | cname | cpno | ccredit | cno | cname | cpno | ccredit |
+-----+--------+------+---------+-----+--------+------+---------+
| 4 | 数据库 | 2 | 4 | 7 | PASCAL | 4 | 4 |
+-----+--------+------+---------+-----+--------+------+---------+
首先引入一个查询块的概念,一个 SELECT...FROM...WHERE... 形式的SQL语句称为查询块。当一个查询块的SELECT子句或者WHERE子句中嵌套了另一个查询块的查询语句就称为嵌套查询。最外层的查询称为外层查询或父查询,最内层的查询称为内层查询或子查询。子查询用到了父查询的数据(表、字段)的情况称为相关子查询,相反,如果没用到就称为不相关子查询。 通常嵌套查询与IN、ALL、ANY、EXISTS配合使用。
-- 查询与刘晨在同一个系中的学生(先查出刘晨所在系,再查该系中的学生)
-- 内层查询可以独立运行没有依赖于外层,所以是不相关子查询
SELECT * FROM student WHERE sdept IN (
SELECT sdept FROM student WHERE sname='刘晨'
)
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
+-----------+-------+------+------+-------+
-- 查询选修了‘信息系统’的学生信息(先查出信息系统的课程号cno,再查处所有选课信息,再查出学生信息)
-- 同样,也是不相关子查询
SELECT * FROM student WHERE sno IN (
SELECT sno FROM sc WHERE cno IN (
SELECT cno FROM course WHERE cname='信息系统'
)
)
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
+-----------+-------+------+------+-------+
-- 找出每个学生超过自己选修课平均成绩的选课信息(先查出平均成绩,再查出选课信息)
-- 内层查询无法独立运行,所以是相关子查询
SELECT * FROM sc AS x WHERE grade >= (
SELECT AVG(grade) FROM sc AS y WHERE x.sno AND y.sno
)
+-----------+------+-------+
| sno | cno | grade |
+-----------+------+-------+
| 201215121 | 1 | 92 |
| 201215121 | 3 | 88 |
| 201215122 | 1 | 90 |
+-----------+------+-------+
个人认为也是嵌套查询的一种,但用得比较广泛,就提出来了。当查询块出现在FROM子句后面时,就称为派生表查询。
-- 查询所有选修了cno=1的课程的学生信息
SELECT * FROM student,(
SELECT sno FROM SC WHERE cno=1
) AS tempSC
WHERE student.sno = tempSC.sno
+-----------+-------+------+------+-------+-----------+
| sno | sname | ssex | sage | sdept | sno |
+-----------+-------+------+------+-------+-----------+
| 201215121 | 李勇 | 男 | 20 | CS | 201215121 |
| 201215122 | 刘晨 | 女 | 19 | CS | 201215122 |
+-----------+-------+------+------+-------+-----------+
涉及UNION、UNION ALL、INTERSECT、EXCEPT的查询操作就称为集合查询。其中,UNION和UNION ALL都会做 并集,但UNION会去除重复的记录。最后,MySQL不支持INTERSECT和EXCEPT。
--查询CS系及年龄不大于19岁的学生(CS系的学生与年龄不大于19岁的学生做并集)
SELECT * FROM student WHERE sdept='CS'
UNION ALL
SELECT * FROM student WHERE sage<=19
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
| 201215123 | 王敏 | 女 | 18 | MA |
| 201215125 | 张立 | 男 | 19 | IS |
+-----------+-------+------+------+-------+
-- UNION去重
SELECT * FROM student WHERE sdept='CS'
UNION
SELECT * FROM student WHERE sage<=19
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
| 201215122 | 刘晨 | 女 | 19 | CS |
| 201215123 | 王敏 | 女 | 18 | MA |
| 201215125 | 张立 | 男 | 19 | IS |
+-----------+-------+------+------+-------+
-- 查询计算机系年龄不大于19岁的学,计算机系的学生与年龄不大于19岁的学生取交集,MySQL不支持INTERSECT操作
SELECT * FROM student WHERE sdept='cs'
INTERSECT
SELECT * FROM student WHERE sage<=19
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'INTERSECT
SELECT * FROM student WHERE sage<=19' at line 2
-- 用内连接代替
SELECT a.* FROM student AS a INNER JOIN student AS b ON a.sno=b.sno
WHERE a.sdept='CS' AND b.sage<=19
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215122 | 刘晨 | 女 | 19 | CS |
+-----------+-------+------+------+-------+
-- 查询计算机系中年龄大于19岁的学生,就是查询计算机系的学生与年龄不大于19岁的学生的差集,MySQL不支持EXCEPT操纵
SELECT * FROM student WHERE sdept='CS'
EXCEPT
SELECT * FROM student WHERE sage<=19
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'EXCEPT
SELECT * FROM student WHERE sage<=19' at line 2
-- 用外连接或普通连接代替
SELECT a.* FROM student AS a LEFT JOIN student AS b ON a.sno=b.sno
WHERE a.sdept='CS'
AND b.sage>19
AND b.sno IS NOT NULL
SELECT * FROM student WHERE sdept='CS' AND sage>19;
+-----------+-------+------+------+-------+
| sno | sname | ssex | sage | sdept |
+-----------+-------+------+------+-------+
| 201215121 | 李勇 | 男 | 20 | CS |
+-----------+-------+------+------+-------+