Advanced SQL

本节介绍 SQL 的部分复杂用法

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

• IBM releases DB2 in 1983

• 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

• Aggregations + Group By

• String / Date / Time Operations

• Output Control + Redirection

• Nested Queries

• Common Table Expressions

• Window Functions

本节使用的示例数据库如下所示：

student(sid, name, login, gpa)

sid	name	login	age	gpa
53666	Kanye	kayne@cs	39	4.0
53668	Bieber	jbieber@cs	22	3.9

enrolled(sid, cid, grade)

sid	cid	grade
53666	15-445	C
53688	15-721	A

course(cid, name)

cid	name
15-445	Database Systems
15-721	Advanced Database Systems

Aggregates

Aggregates 通常返回一个值，它们包括：

• AVG(col)

• MIN(col)

• MAX(col)

• SUM(col)

• COUNT(col)

举例如下：

count.sql

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';

multiple.sql

SELECT AVG(gpa), COUNT(sid)
  FROM student WHERE login LIKE '%@cs';

distinct.sql

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：

mix.sql

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：

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：

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：

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：

• 该表必须是已经存在的表

• 该表的列数，以及每列的数据类型必须相同

SQL-92.sql

SELECT DISTINCT cid INTO CourseIds
  FROM enrolled;

INSERT INTO CourseIds (
    SELECT DISTINCT cid FROM enrolled
);

MySQL.sql

Output Control

Order By

语法：ORDER BY <column*> [ASC|DESC]

SELECT sid, grade FROM enrolled
 WHERE cid = '15-721'
 ORDER BY grade;

按多个字段分别排序：

SELECT sid FROM enrolled
 WHERE cid = '15-721'
 ORDER BY grade DESC, sid ASC;

Limit

语法：LIMIT <count> [offset]

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：

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

• IN：与 ANY 等价

• EXISTS：inner queries 返回的表不为空

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

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

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：

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

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

WITH RECURSIVE cteSource (counter) AS (
    (SELECT 1)
    UNION ALL
    (SELECT counter + 1 FROM cteSource
      WHERE counter < 10)
)
SELECT * FROM cteSource;

String Operations

DBMS	String Case	String Quotes
SQL-92	Sensitive	Single Only
Postgres	Sensitive	Single Only
MySQL	InSensitive	Single/Double
SQLite	Sensitive	Single/Double
Oracle	Sensitive	Single Only

如在 condition 中判断两个字符串忽略大小写后是否相等：

string-equal.sql

/* SQL-92 */
WHERE UPPER(name) = UPPER('KaNyE')

/* MySQL */
WHERE name = "KaNyE"

String Matching

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

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

concatenation.sql

/* 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：

SELECT *, ROW_NUMBER() OVER () AS row_num
  FROM enrolled;

会得到类似下表：

window functions - example1

例 2：

SELECT cid, sid,
       ROW_NUMBER() OVER (PARTITION BY cid)
  FROM enrolled
 ORDER BY cid;

可是得到类似下表：

window functions - example 2

例 3：找到每门课获得最高分的学生

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