Select
用途:
从指定表中取出指定的列的数据
语法:
SELECT column_name(s) FROM table_name
解释:
从数据库中选取资料列,并允许从一或多个资料表中,选取一或多个资料列或资料行。SELECT 陈述式的完整语法相当复杂,但主要子句可摘要为:
SELECT select_list
[ INTO new_table ]
FROM table_source
[ WHERE search_condition ]
[ GROUP BY group_by_expression ]
[ HAVING search_condition ]
[ ORDER BY order_expression [ ASC | DESC ] ]
例:
“Persons” 表中的数据有
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
选出字段名”
LastName”、”
FirstName” 的数据
SELECT LastName,FirstName FROM Persons
返回结果:
LastName |
FirstName |
Hansen |
Ola |
Svendson |
Tove |
Pettersen |
Kari |
选出所有字段的数据
SELECT * FROM Persons
返回结果:
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
Where
用途:
被用来规定一种选择查询的标准
语法:
SELECT column FROM table WHERE column condition value
=,<>,>,<,>=,<=,BETWEEN,LIKE下面的操作符能被使用在WHERE中:
注意: 在某些SQL的版本中不等号< >能被写作为!=
解释:
SELECT语句返回WHERE子句中条件为true的数据
例:
从”Persons”表中选出生活在” Sandnes” 的人
SELECT * FROM Persons WHERE City='Sandnes'
"Persons" 表中的数据有:
LastName |
FirstName |
Address |
City |
Year |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
1951 |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
1978 |
Svendson |
Stale |
Kaivn 18 |
Sandnes |
1980 |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
1960 |
返回结果:
LastName |
FirstName |
Address |
City |
Year |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
1951 |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
1978 |
Svendson |
Stale |
Kaivn 18 |
Sandnes |
1980 |
And & Or
用途:
在WHERE子句中AND和OR被用来连接两个或者更多的条件
解释:
AND在结合两个布尔表达式时,只有在两个表达式都为 TRUE 时才传回 TRUE
OR在结合两个布尔表达式时,只要其中一个条件为 TRUE 时,OR便传回 TRUE
例:
"Persons" 表中的原始数据:
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Svendson |
Stephen |
Kaivn 18 |
Sandnes |
用AND运算子来查找"Persons" 表中FirstName为”Tove”而且LastName为” Svendson”的数据
SELECT * FROM Persons
WHERE FirstName='Tove'
AND LastName='Svendson'
返回结果:
LastName |
FirstName |
Address |
City |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
用OR运算子来查找"Persons" 表中FirstName为”Tove”或者LastName为” Svendson”的数据
SELECT * FROM Persons
WHERE firstname='Tove'
OR lastname='Svendson'
返回结果:
LastName |
FirstName |
Address |
City |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Svendson |
Stephen |
Kaivn 18 |
Sandnes |
你也能结合AND和OR (使用括号形成复杂的表达式),如:
SELECT * FROM Persons WHERE
(FirstName='Tove' OR FirstName='Stephen')
AND LastName='Svendson'
返回结果:
LastName |
FirstName |
Address |
City |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Svendson |
Stephen |
Kaivn 18 |
Sandnes |
Between…And
用途:
指定需返回数据的范围
语法:
SELECT column_name FROM table_name
WHERE column_name
BETWEEN value1AND value2
“Persons”表中的原始数据例:
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Nordmann |
Anna |
Neset 18 |
Sandnes |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
用BETWEEN…AND返回LastName为从”Hansen”到”Pettersen”的数据:
SELECT * FROM Persons WHERE LastName
BETWEEN 'Hansen' AND 'Pettersen'
返回结果:
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Nordmann |
Anna |
Neset 18 |
Sandnes |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
为了显示指定范围之外的数据,也可以用NOT操作符:
SELECT * FROM Persons WHERE LastName
NOT BETWEEN 'Hansen' AND 'Pettersen'
返回结果:
LastName |
FirstName |
Address |
City |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Distinct
用途:
DISTINCT关键字被用作返回唯一的值
语法:
SELECT DISTINCT column-name(s) FROM table-name
当column-name(s)中存在重复的值时,返回结果仅留下一个解释:
例:
“Orders”表中的原始数据
Company |
OrderNumber |
Sega |
3412 |
W3Schools |
2312 |
Trio |
4678 |
W3Schools |
6798 |
用DISTINCT关键字返回Company字段中唯一的值:
SELECT DISTINCT Company FROM Orders
返回结果:
Company |
Sega |
W3Schools |
Trio |
Order by
用途:
指定结果集的排序
语法:
SELECT column-name(s) FROM table-name ORDER BY { order_by_expression [ ASC | DESC ] }
指定结果集的排序,可以按照ASC(递增方式排序,从最低值到最高值)或者DESC(递减方式排序,从最高值到最低值)的方式进行排序,默认的方式是ASC解释:
例:
“Orders”表中的原始数据:
Company |
OrderNumber |
Sega |
3412 |
ABC Shop |
5678 |
W3Schools |
2312 |
W3Schools |
6798 |
按照Company字段的升序方式返回结果集:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company
返回结果:
Company |
OrderNumber |
ABC Shop |
5678 |
Sega |
3412 |
W3Schools |
6798 |
W3Schools |
2312 |
按照Company字段的降序方式返回结果集:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company DESC
返回结果:
Company |
OrderNumber |
W3Schools |
6798 |
W3Schools |
2312 |
Sega |
3412 |
ABC Shop |
5678 |
Group by
用途:
对结果集进行分组,常与汇总函数一起使用。
语法:
SELECT column,SUM(column) FROM table GROUP BY column
“Sales”表中的原始数据:例:
Company |
Amount |
W3Schools |
5500 |
IBM |
4500 |
W3Schools |
7100 |
按照Company字段进行分组,求出每个Company的Amout的合计:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
返回结果:
Company |
SUM(Amount) |
W3Schools |
12600 |
IBM |
4500 |
Having
用途:
指定群组或汇总的搜寻条件。
语法:
SELECT column,SUM(column) FROM table
GROUP BY column
HAVING SUM(column) condition value
HAVING 通常与 GROUP BY 子句同时使用。不使用 GROUP BY 时,HAVING 则与 WHERE 子句功能相似。解释:
例:
“Sales”表中的原始数据:
Company |
Amount |
W3Schools |
5500 |
IBM |
4500 |
W3Schools |
7100 |
按照Company字段进行分组,求出每个Company的Amout的合计在10000以上的数据:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company HAVING SUM(Amount)>10000
返回结果:
Company |
SUM(Amount) |
W3Schools |
12600 |
Join
用途:
当你要从两个或者以上的表中选取结果集时,你就会用到JOIN。
例:
“Employees”表中的数据如下,(其中ID为主键):
ID |
Name |
01 |
Hansen, Ola |
02 |
Svendson, Tove |
03 |
Svendson, Stephen |
04 |
Pettersen, Kari |
“Orders”表中的数据如下:
ID |
Product |
01 |
Printer |
03 |
Table |
03 |
Chair |
用Employees的ID和Orders的ID相关联选取数据:
SELECT Employees.Name, Orders.Product
FROM Employees, Orders
WHERE Employees.ID = Orders.ID
返回结果:
Name |
Product |
Hansen, Ola |
Printer |
Svendson, Stephen |
Table |
Svendson, Stephen |
Chair |
或者你也可以用JOIN关键字来完成上面的操作:
SELECT Employees.Name, Orders.Product
FROM Employees
INNER JOIN Orders
ON Employees.ID = Orders.ID
INNER JOIN的语法:
SELECT field1, field2, field3
FROM first_table
INNER JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
INNER JOIN返回的结果集是两个表中所有相匹配的数据。解释:
LEFT JOIN的语法:
SELECT field1, field2, field3
FROM first_table
LEFT JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
用”Employees”表去左外联结”Orders”表去找出相关数据:
SELECT Employees.Name, Orders.Product
FROM Employees
LEFT JOIN Orders
ON Employees.ID = Orders.ID
返回结果:
Name |
Product |
Hansen, Ola |
Printer |
Svendson, Tove |
|
Svendson, Stephen |
Table |
Svendson, Stephen |
Chair |
Pettersen, Kari |
解释:
LEFT
JOIN返回”first_table”中所有的行尽管在”
second_table”中没有相匹配的数据。
RIGHT
JOIN的语法:
SELECT field1, field2, field3
FROM first_table
RIGHT JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
用”Employees”表去右外联结”Orders”表去找出相关数据:
SELECT Employees.Name, Orders.Product
FROM Employees
RIGHT JOIN Orders
ON Employees.ID = Orders.ID
返回结果:
Name |
Product |
Hansen, Ola |
Printer |
Svendson, Stephen |
Table |
Svendson, Stephen |
Chair |
解释:
RIGHT JOIN返回” second_table”中所有的行尽管在”first_table”中没有相匹配的数据。
Alias
用途:
可用在表、结果集或者列上,为它们取一个逻辑名称
语法:
给列取别名:
SELECT column AS column_alias FROM table
给表取别名:
SELECT column FROM table AS table_alias
“Persons”表中的原始数据:例:
LastName |
FirstName |
Address |
City |
Hansen |
Ola |
Timoteivn 10 |
Sandnes |
Svendson |
Tove |
Borgvn 23 |
Sandnes |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
运行下面的SQL:
SELECT LastName AS Family, FirstName AS Name
FROM Persons
返回结果:
Family |
Name |
Hansen |
Ola |
Svendson |
Tove |
Pettersen |
Kari |
运行下面的SQL:
SELECT LastName, FirstName
FROM Persons AS Employees
Employees中的数据有:返回结果:
LastName |
FirstName |
Hansen |
Ola |
Svendson |
Tove |
Pettersen |
Kari |
Insert Into
用途:
在表中插入新行
语法:
插入一行数据
INSERT INTO table_name
VALUES (value1, value2,....)
插入一行数据在指定的字段上
INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,....)
“Persons”表中的原始数据:例:
LastName |
FirstName |
Address |
City |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
运行下面的SQL插入一行数据:
INSERT INTO Persons
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
插入后”Persons”表中的数据为:
LastName |
FirstName |
Address |
City |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
Hetland |
Camilla |
Hagabakka 24 |
Sandnes |
运行下面的SQL插入一行数据在指定的字段上:
INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
插入后”Persons”表中的数据为:
LastName |
FirstName |
Address |
City |
Pettersen |
Kari |
Storgt 20 |
Stavanger |
Hetland |
Camilla |
Hagabakka 24 |
Sandnes |
Rasmussen |
Storgt 67 |
Update
用途:
更新表中原有数据
语法:
UPDATE table_nameSET column_name = new_value
WHERE column_name = some_value
“Person”表中的原始数据:例:
LastName |
FirstName |
Address |
City |
Nilsen |
Fred |
Kirkegt 56 |
Stavanger |
Rasmussen |
Storgt 67 |
运行下面的SQL将Person表中LastName字段为”Rasmussen”的FirstName更新为”Nina”:
UPDATE Person SET FirstName = 'Nina'
WHERE LastName = 'Rasmussen'
更新后”Person”表中的数据为:
LastName |
FirstName |
Address |
City |
Nilsen |
Fred |
Kirkegt 56 |
Stavanger |
Rasmussen |
Nina |
Storgt 67 |
同样的,用UPDATE语句也可以同时更新多个字段:
UPDATE Person
SET Address = 'Stien 12', City = 'Stavanger'
WHERE LastName = 'Rasmussen'
更新后”Person”表中的数据为:
LastName |
FirstName |
Address |
City |
Nilsen |
Fred |
Kirkegt 56 |
Stavanger |
Rasmussen |
Nina |
Stien 12 |
Stavanger |
Delete
用途:
删除表中的数据
语法:
DELETE FROM table_nameWHERE column_name = some_value
“Person”表中的原始数据:例:
LastName |
FirstName |
Address |
City |
Nilsen |
Fred |
Kirkegt 56 |
Stavanger |
Rasmussen |
Nina |
Stien 12 |
Stavanger |
删除Person表中LastName为”Rasmussen”的数据:
DELETE FROM Person WHERE LastName = 'Rasmussen'
执行删除语句后”Person”表中的数据为:
LastName |
FirstName |
Address |
City |
Nilsen |
Fred |
Kirkegt 56 |
Stavanger |
Create Table
用途:
建立新的资料表。
语法:
CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
.......
)
创建一张叫“Person”的表,该表有4个字段"LastName", "FirstName", "Address", "Age":例:
CREATE TABLE Person
(
LastName varchar,
FirstName varchar,
Address varchar,
Age int
)
如果想指定字段的最大存储长度,你可以这样:
CREATE TABLE Person
(
LastName varchar(30),
FirstName varchar(30),
Address varchar(120),
Age int(3)
)
下表中列出了在SQL的一些数据类型:
Data Type |
Description |
integer(size) |
Hold integers only. The maximum number of digits are specified in |
decimal(size,d) |
Hold numbers with fractions. The maximum number of digits are specified |
char(size) |
Holds a fixed length string (can contain letters, numbers, and special |
varchar(size) |
Holds a variable length string (can contain letters, numbers, and |
date(yyyymmdd) |
Holds a date |
Alter Table
用途:
在已经存在的表中增加后者移除字段
语法:
ALTER TABLE table_name
ADD column_name datatype
ALTER TABLE table_name
DROP COLUMN column_name
注意:某些数据库管理系统不允许移除表中的字段
例:
“Person”表中的原始数据:
LastName |
FirstName |
Address |
Pettersen |
Kari |
Storgt 20 |
在Person表中增加一个名为City的字段:
ALTER TABLE Person ADD City varchar(30)
增加后表中数据如下:
LastName |
FirstName |
Address |
City |
Pettersen |
Kari |
Storgt 20 |
移除Person表中原有的Address字段:
ALTER TABLE Person DROP COLUMN Address
移除后表中数据如下:
LastName |
FirstName |
City |
Pettersen |
Kari |
Drop Table
用途:
在数据库中移除一个数据表定义及该数据表中的所有资料、索引、触发程序、条件约束及权限指定。
语法:
DROP TABLE table_name
Create Database
用途:
建立新的数据库.
语法:
CREATE DATABASE database_name
Drop Database
用途:
移除原有的数据库
语法:
DROP DATABASE database_name
聚集函数
count
用途:
传回选取的结果集中行的数目。
语法:
SELECT COUNT(column_name) FROM table_name
“Persons”表中原始数据如下:例:
Name |
Age |
Hansen, Ola |
34 |
Svendson, Tove |
45 |
Pettersen, Kari |
19 |
选取记录总数:
SELECT COUNT(Name) FROM Persons
执行结果:
3 |
sum
用途:
以表达式传回所有值的总和,或仅 DISTINCT 值。SUM 仅可用于数值资料行。已忽略 Null 值。
语法:
SELECT SUM(column_name) FROM table_name
“Persons”表中原始数据如下:例:
Name |
Age |
Hansen, Ola |
34 |
Svendson, Tove |
45 |
Pettersen, Kari |
19 |
选取”Persons”表中所有人的年龄总和:
SELECT SUM(Age) FROM Persons
执行结果:
98 |
选取”Persons”表中年龄超过20岁的人的年龄总和:
SELECT SUM(Age) FROM Persons WHERE Age>20
执行结果:
79 |
avg
用途:
传回选取的结果集中值的平均值。已忽略 Null 值。
语法:
SELECT AVG(column_name) FROM table_name
“Persons”表中原始数据如下:例:
Name |
Age |
Hansen, Ola |
34 |
Svendson, Tove |
45 |
Pettersen, Kari |
19 |
选取”Persons”表中所有人的平均年龄:
SELECT AVG(Age) FROM Persons
执行结果:
32.67 |
选取”Persons”表中年龄超过20岁的人的平均年龄:
SELECT AVG(Age) FROM Persons WHERE Age>20
执行结果:
39.5 |
max
用途:
传回选取的结果集中值的最大值。已忽略 Null 值。
语法:
SELECT MAX(column_name) FROM table_name
“Persons”表中原始数据如下:例:
Name |
Age |
Hansen, Ola |
34 |
Svendson, Tove |
45 |
Pettersen, Kari |
19 |
选取”Persons”表中的最大年龄:
SELECT MAX(Age) FROM Persons
执行结果:
45 |
min
用途:
传回选取的结果集中值的最小值。已忽略 Null 值。
语法:
SELECT MIN(column_name) FROM table_name
“Persons”表中原始数据如下:例:
Name |
Age |
Hansen, Ola |
34 |
Svendson, Tove |
45 |
Pettersen, Kari |
19 |
选取”Persons”表中的最小年龄:
SELECT MIN(Age) FROM Persons
执行结果:
19 |
算术函数
abs
用途:
传回指定数值表达式 (Numeric Expression) 的绝对正值。
语法:
ABS(numeric_expression)
例:
|
执行结果:
|
ceil
用途:
传回大于等于给定数值表达式的最小整数。
语法:
CEIL(numeric_expression)
例:
|
执行结果:
|
floor
用途:
传回小于或等于给定数值表达式的最大整数。
语法:
FLOOR(numeric_expression) |
例:
|
执行结果:
|
cos
用途:
在指定表达式中传回指定角度 (以弪度为单位) 的三角余弦值的数学函数。
语法:
COS(numeric_expression) |
例:
|
执行结果:
-0.599465 |
cosh
用途:
传回以弧度为单位的角度值,其余弦为指定的 float 表达式,也称为反余弦。
语法:
COSH(numeric_expression) |
例:
|
执行结果:
3.14159 |
sin
用途:
以近似的数值 (float) 表达式传回给定角度 (以弧度) 之三角正弦函数 (Trigonometric Sine)。
语法:
SIN(numeric_expression) |
例:
|
执行结果:
|
sinh
用途:
传回以弪度为单位的角度,其正弦为指定的 float 表达式 (也称为反正弦)。
语法:
SINH(numeric_expression) |
例:
|
执行结果:
|
tan
用途:
传回输入表达式的正切函数。
语法:
TAN(numeric_expression) |
例:
|
执行结果:
|
tanh
用途:
传回以弪度为单位的角度,其正切为指定的 float 表达式 (也称为反正切)。
语法:
TANH(numeric_expression) |
例:
|
执行结果:
|
exp
用途:
传回给定的 float 表达式的指数 (Exponential) 值。
语法:
EXP(numeric_expression) |
例:
|
执行结果:
|
log
用途:
传回给定的 float 表达式之自然对数。
语法:
LOG(numeric_expression) |
例:
|
执行结果:
|
power
用途:
传回给定表达式指定乘幂的值。
语法:
POWER(numeric_expression,v) |
例:
|
执行结果:
|
sign
用途:
传回给定的表达式之正 (+1)、零 (0) 或负 (-1) 号。
语法:
SIGN(numeric_expression) |
例:
|
执行结果:
|
sqrt
用途:
传回给定表达式的平方。
语法:
SQRT(numeric_expression) |
例:
|
执行结果:
|