SQL 语法
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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
下面的操作符能被使用在WHERE中:
=,<>,>,<,>=,<=,BETWEEN,LIKE
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 PersonsWHERE 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_nameWHERE column_nameBETWEEN value1 AND value2
例:
“Persons”表中的原始数据
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
解释:
当column-name(s)中存在重复的值时,返回结果仅留下一个
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 ] }
解释:
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
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 OrdersORDER 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 OrdersORDER 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
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:
SELECT Company,SUM(Amount) FROM SalesGROUP 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
解释:
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。
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.ProductFROM 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:
SELECT field1, field2, field3 FROM first_table INNER JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield
解释:
INNER JOIN返回的结果集是两个表中所有相匹配的数据。
LEFT JOIN:
SELECT field1, field2, field3FROM first_tableLEFT JOIN second_tableON 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.ProductFROM EmployeesLEFT JOIN OrdersON 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:
SELECT field1, field2, field3FROM first_tableRIGHT JOIN second_tableON 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.ProductFROM EmployeesRIGHT JOIN OrdersON 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。
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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
例:
“Persons”表中的原始数据:
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
返回结果:
Employees中的数据有:
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,....)
例:
“Persons”表中的原始数据:
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_name SET column_name = new_valueWHERE column_name = some_value
例:
“Person”表中的原始数据:
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 PersonSET 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_name WHERE column_name = some_value
例:
“Person”表中的原始数据:
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,.......)
例:
创建一张叫“Person”的表,该表有4个字段"LastName", "FirstName", "Address", "Age":
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) int(size) smallint(size) tinyint(size) | Hold integers only. The maximum number of digits are specified in parenthesis. |
decimal(size,d) numeric(size,d) | Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d". |
char(size) | Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis. |
varchar(size) | Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis. |
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 datatypeALTER 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
例:
“Persons”表中原始数据如下:
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
例:
“Persons”表中原始数据如下:
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
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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
例:
“Persons”表中原始数据如下:
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
例:
“Persons”表中原始数据如下:
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
例:
“Persons”表中原始数据如下:
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:
ABS(-1.0) ABS(0.0) ABS(1.0)
执行结果:
1.0 0.0 1.0
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