回顾
在 Relational Model 下构建查询语句的方式分为两种:Procedural 和 Non-Procedural。第一节课中已经介绍了 Relational Algebra,它属于 Procedural 类型,而本节将介绍的 SQL 属于 Non-Procedural 类型。使用 SQL 构建查询时,用户只需要指定它们想要的数据,而不需要关心数据获取的方式,DBMS 负责理解用户的查询语义,选择最优的方式构建查询计划。
SQL 的历史
”SEQUAL" from IBM's System R prototype
Structured English Query Language
Adopted by Oracle in the 1970s
ANSI Standard in 1986. ISO in 1987
Structured Query Language
当前 SQL 的标准是 SQL 2016,而目前大部分 DBMSs 至少支持 SQL-92 标准,具体的系统对比信息可以到这里 查询。
SQL 主要特性介绍
SQL 基于
概要
String / Date / Time Operations
Output Control + Redirection
本节使用的示例数据库如下所示:
student(sid, name, login, gpa)
enrolled(sid, cid, grade)
course(cid, name)
Aggregates
Aggregates 通常返回一个值,它们包括:
举例如下:
Copy SELECT COUNT ( login ) AS cnt
FROM student WHERE login LIKE '%@cs' ;
SELECT COUNT ( * ) AS cnt
FROM student WHERE login LIKE '%@cs' ;
SELECT COUNT ( 1 ) AS cnt
FROM student WHERE login LIKE '%@cs' ;
Copy SELECT AVG (gpa), COUNT ( sid )
FROM student WHERE login LIKE '%@cs' ;
Copy SELECT COUNT ( DISTINCT login )
FROM student WHERE login LIKE '%@cs' ;
aggregate 与其它通常的查询列不可以混用,比如:
Copy SELECT AVG (s.gpa), e.cid
FROM enrolled AS e, student AS s
WHERE e.sid = s.sid;
不同 DBMSs 的输出结果不一样,严格模式下,DBMS 应该抛错。
Group By
group by 就是把记录按某种方式分成多组,对每组记录分别做 aggregates 操作,如求每门课所有学生的 GPA 平均值:
Copy SELECT AVG (s.gpa), e.cid
FROM enrolled AS e, student AS s
WHERE e.sid = s.sid
GROUP BY e.cid;
所有非 aggregates 操作的字段,都必须出现在 group by 语句,如下面示例中的 e.cid 和 s.name:
Copy SELECT AVG (s.gpa), e.cid, s.name
FROM enrolled AS e, student AS s
WHERE e.sid = s.sid
GROUP BY e.cid, s.name;
Having
基于 aggregation 结果的过滤条件不能写在 WHERE 中,而应放在 HAVING 中,如:
Copy SELECT AVG (s.gpa) AS avg_gpa, e.cid
FROM enrolled AS e, student AS s
WHERE e.sid = s.sid
GROUP BY e.cid
HAVING avg_gpa > 3 . 9 ;
Output Redirection
将查询结果储存到另一张表上:
SQL-92.sql MySQL.sql
Copy SELECT DISTINCT cid INTO CourseIds
FROM enrolled;
INSERT INTO CourseIds (
SELECT DISTINCT cid FROM enrolled
);
Copy CREATE TABLE CourseIds (
SELECT DISTINCT cid FROM enrolled
);
Output Control
Order By
语法:ORDER BY <column*> [ASC|DESC]
Copy SELECT sid , grade FROM enrolled
WHERE cid = '15-721'
ORDER BY grade;
按多个字段分别排序:
Copy SELECT sid FROM enrolled
WHERE cid = '15-721'
ORDER BY grade DESC , sid ASC ;
Limit
语法:LIMIT <count> [offset]
Copy SELECT sid , name FROM student
WHERE login LIKE '%@cs'
LIMIT 10 ;
SELECT sid , name FROM student
WHERE login LIKE '%@cs'
LIMIT 20 OFFSET 10 ;
Nested Queries
nested queries 包含 inner queries 和 outer queries,前者可以出现在 query 的任何位置,且 inner queries 可以引用 outer queries 中的表信息。
例 1:获取所有参与 '15-445' 这门课所有学生的姓名:
Copy SELECT name FROM student
WHERE sid IN (
SELECT sid FROM enrolled
WHERE cid = '15-445'
);
SELECT ( SELECT S.name FROM student AS S
WHERE S.sid = E.sid) AS sname
FROM enrolled AS E
WHERE cid = '15-445' ;
语法中支持的谓词包括:
ALL: 所有 inner queries 返回的记录都必须满足条件
ANY :任意 inner queries 返回的记录满足条件即可
EXISTS: inner queries 返回的表不为空
Copy SELECT name FROM student
WHERE sid ANY (
SELECT sid FROM enrolled
WHERE cid = '15-445'
)
例 2:找到至少参与一门课程的所有学生中,id 最大的
Copy SELECT sid , name FROM student
WHERE sid >= ALL (
SELECT sid FROM enrolled
);
SELECT sid , name FROM student
WHERE sid IN (
SELECT MAX ( sid ) FROM enrolled
);
SELECT sid , name FROM student
WHERE sid IN (
SELECT sid FROM enrolled
ORDER BY sid DESC LIMIT 1
);
例 3:找到所有没有学生参与的课程
Copy SELECT * FROM course
WHERE NOT EXISTS (
SELECT * FROM enrolled
WHERE course.cid = enrolled.cid
);
nested queries 比较难被优化(具体原因暂不知道)
Common Table Expressions
在一些复杂查询中,创建一些中间表能够使得这些查询逻辑更加清晰:
Copy WITH cteName AS (
SELECT 1
)
SELECT * FROM cteName
WITH cteName (col1, col2) AS (
SELECT 1 , 2
)
SELECT col1 + col2 FROM cteName
WITH
cteName1(col) AS ( SELECT 1 ),
cteName2(col) AS ( SELECT 2 )
SELECT C1.col + C2.col FROM cteName1 AS C1, cteName2 AS C2;
例 1:找到所有参与课程的学生中 id 最大的
Copy WITH cteSource(maxId) AS (
SELECT MAX ( sid ) FROM enrolled
)
SELECT name FROM student, cteSource
WHERE student.sid = cteSource.maxId
例 2:打印 1-10
Copy WITH RECURSIVE cteSource ( counter ) AS (
( SELECT 1 )
UNION ALL
( SELECT counter + 1 FROM cteSource
WHERE counter < 10 )
)
SELECT * FROM cteSource;
String Operations
如在 condition 中判断两个字符串忽略大小写后是否相等:
Copy /* SQL-92 */
WHERE UPPER ( name ) = UPPER ( 'KaNyE' )
/* MySQL */
WHERE name = "KaNyE"
String Matching
Copy SELECT * FROM enrolled AS e
WHERE e.cid LIKE '15-%' ;
SELECT * FROM student AS s
WHERE s.login LIKE '%@c_' ;
String Operations
SQL-92 定义了一些 string 函数,如
Copy SELECT SUBSTRING ( name , 0 , 5 ) AS abbrv_name
FROM student WHERE sid = 53688 ;
SELECT * FROM student AS s
WHERE UPPER (e.name) LIKE 'KAN%' ;
不同 DBMS 有不同的 string 函数(没有完全遵从 SQL-92 标准),如连接两个 strings
Copy /* SQL-92 */
SELECT name FROM student
WHERE login = LOWER ( name ) || '@cs' ;
/* MySQL */
SELECT name FROM student
WHERE login = LOWER ( name ) + '@cs' ;
SELECT name FROM student
WHERE login = CONCAT ( LOWER ( name ), '@cs' )
Date/Time Operations
不同的 DBMS 中的定义和用法不一样,具体见各 DBMS 的文档。
Window Functions
主要用于在一组记录中,对每一条记录进行一些计算,如:
例 1:
Copy SELECT * , ROW_NUMBER () OVER () AS row_num
FROM enrolled;
会得到类似下表:
例 2:
Copy SELECT cid, sid ,
ROW_NUMBER () OVER ( PARTITION BY cid)
FROM enrolled
ORDER BY cid;
可是得到类似下表:
例 3:找到每门课获得最高分的学生
Copy SELECT * FROM (
SELECT * ,
RANK () OVER ( PARTITION BY cid ORDER BY grade ASC ) AS rank
FROM enrolled
) AS ranking
WHERE ranking.rank = 1
参考
slide , video