SELECT attribute-expression-list FROM relation-list [ WHERE condition ];

attribute-expression-list:

• [relation-name.]attribute
• [relation-name.]attribute [arithmatic computation] AS another name
  
  
  • i.e., E.Salary - 40000 AS SalaryDiff
• CASE WHEN ... THEN ...
  ELSE ... END
  
  
  • i.e., CASE WHEN E.Salary < 40000 THEN 0
    ELSE E.Salary - 40000 END

relation-list:

• list of table names
• seperate by comma

condition:

• arithmetic operation +, -, *, /
• comparisions =, <>, <, <=, >. >=
• logical connectives AND, OR, NOT
• attribute IN (Q)
• attribute NOT IN (Q)
• attribute op SOME (Q)
• attribute op ALL (Q)
• EXISTS (Q)
• NOT EXISTS (Q)
• IS [NOT] NULL

NULL

AND Table

OR Table

NOT Table

UNION INTERSECT EXCEPT

(Q1 and Q2 must have same attribute-list)

Q1 UNION Q2 => Together all the tuples in Q1 and Q2
Q1 INTERSECT Q2 => Only tuples in both Q1 and Q2
Q1 EXCEPT Q2 => Tuples only in Q1 but not in Q2

ALL关键字: 允许重复
UNION ALL will include twice of duplicate tuples
INTERSECT ALL will include all possible pairs of match tuples, duplication possible
EXCEPT ALL will include all "not in Q2" tuples, duplication possible

[INNER]/OUTER JOIN

1. Inner join把两个表连接在一起, 返回两个表中相匹配的记录, 是2和3的交集
2. Left outer join, 左侧表所有的记录都返回, 右侧匹配的记录返回, 没有匹配的返回Null
3. Right outer join, 与Left outer join相反, 右侧的记录返回, 左侧返回匹配的记录, 没有匹配返回Null
4. Full outer join, 2和3的并集
5. Cross join, 两个表的笛卡儿积, 返回所有可能的值, 不允许有连接条件

ORDER BY

SELECT ... ... ORDER BY attribute [DESC/ASC], attribute [DESC/ASC], ......

Note: 如果没有指定Order, return的数据可能是任意顺序

GROUP BY, HAVING, Aggregate expressions

{count, sum, avg, min, max} => Aggregate expressions

Order: Group => Having => Aggregate

• count(*): number of tuples
• count(E): number of tupple for which E is non-NULL
• count(distinct E): number of distinct non-NULL E values
• sum(E)
• sum(distinct E)
• avg(E)
• avg(distinct E)
• min(E)
• max(E)

Note: 没有被group by指定的attribute不能出现在SELECT的attr-list中, 除非是aggregate

INSERT INTO

INSERT INTO relation-name [( attribute-list )] VALUE ( value-list );

DELETE

DELETE FROM relation-name [ WHERE condition ];

UPDATE

UPDATE relation-name SET attribute-assignment-list [ WHERE condition ]; 

attribute-assignment-list:

• pairs of assignment
• seperate by comma
• i.e., WorkDept = 'E01', Address = 'Waterloo'

CREATE TABLE

CREATE TABLE relation-name ( attribute-name attribute-type [constraints-list], ... )

attribute-type: http://www.w3school.com.cn/sql/sql_datatypes.asp

constraints-list: (Constraints的格式在各种数据库中都不太一样, 就不列举了)

• NOT NULL
• PRIMARY KEY
• UNIQUE
• FOREIGN KEY
• Column or Tuple CHECK

CREATE VIEW

CREATE VIEW view-name AS ( SELECT ... ) 

从View SELECT的方法和table一样

CREATE TRIGGER

CREATE TRIGGER trigger-name 
AFTER UPDATE OF attribute-list ON relation-name 
REFERENCING OLD as instance-name(o) NEW as instance-name(n) 
FOR EACH ROW ...  

不同database语法不一