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
SELECT DISTINCT cid INTO CourseIds
FROM enrolled;
INSERT INTO CourseIds (
SELECT DISTINCT cid FROM enrolled
);
CREATE TABLE CourseIds (
SELECT DISTINCT cid FROM enrolled
);
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;
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%';
/* 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:
例 2:
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
SELECT * FROM (
SELECT *,
RANK() OVER (PARTITION BY cid ORDER BY grade ASC) AS rank
FROM enrolled
) AS ranking
WHERE ranking.rank = 1