SQL 是一门 ANSI 的标准计算机语言,用来访问和操作数据库系统。有些命令不同的RDBMS中不一致,以MySQL为例。以下是整理的一些常用的sql基础语句:
DATABASE
-- 创建新数据库
CREATE DATABASE my_db
-- 修改数据库
ALTER DATABASE my_db CHARACTER SET utf8
-- 重命名数据库
RENAME DATABASE my_db TO my_db1
-- 删除数据库
DROP DATABASE my_db1
TABLE
-- 创建表
CREATE TABLE Persons (
Id_P int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes',
)
-- 重命名
RENAME TABLE Person TO Student
-- 添加列
ALTER TABLE Person ADD Age int
-- 删除列
ALTER TABLE Person DROP COLUMN Age
-- 改变列的数据类型
ALTER TABLE Person ALTER COLUMN Age varchar(255)
-- 删除表数据
TRUNCATE TABLE Person
-- 删除表
DROP TABLE Person
SELECT
-- 从表中选择列
SELECT LastName,FirstName FROM Persons
-- 只列出不同的值
SELECT DISTINCT Company FROM Orders
-- 指定数目
SELECT * FROM Persons LIMIT 5
-- 排序,默认ASC
SELECT Company, OrderNumber FROM Orders ORDER BY Company
SELECT Company, OrderNumber FROM Orders ORDER BY Company DESC, OrderNumber ASC
-- 表/列别名
SELECT po.OrderID, p.LastName, p.FirstName FROM Persons AS p, Product_Orders AS po
WHERE p.LastName='Adams' AND p.FirstName='John'
SELECT LastName AS Family, FirstName AS Name FROM Persons
--- 备份至指定表
SELECT LastName,FirstName INTO Persons_backup FROM Persons
--- 备份至指定数据库
SELECT * INTO Persons IN 'Backup.mdb' FROM Persons
WHERE
-- 条件选取
SELECT * FROM Persons WHERE City='Beijing'
SELECT * FROM Persons WHERE FirstName='Thomas' AND LastName='Carter'
-- 模式匹配
SELECT * FROM Persons WHERE City LIKE 'N%'
-- 多值匹配
SELECT * FROM Persons WHERE LastName IN ('Adams','Carter')
-- 范围匹配
SELECT * FROM Persons WHERE LastName BETWEEN 'Adams' AND 'Carter'
SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Adams' AND 'Carter'
SQL通配符:
通配符 | 描述 |
---|---|
% | 替代一个或多个字符 |
_ | 仅替代一个字符 |
[charlist] | 字符列中的任何单一字符 |
[^charlist] 或 [!charlist] | 不在字符列中的任何单一字符 |
JOIN/UNION
-- 引用两个表
SELECT Persons.FirstName, Orders.OrderNo FROM Persons, Orders WHERE Persons.Id_P = Orders.Id_P
-- Join,结果同上
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons
INNER JOIN Orders
ON Persons.Id_P = Orders.Id_P ORDER BY Persons.LastName
--- Union,结果合并,要求相同数量的列,兼容的数据类型。
SELECT E_Name FROM Employees_China
UNION
SELECT E_Name FROM Employees_USA
--- Union all,允许重复
SELECT E_Name FROM Employees_China
UNION ALL
SELECT E_Name FROM Employees_USA
操作符 | 描述 |
---|---|
(INNER) JOIN | 如果表中有至少一个匹配,则返回行 |
LEFT JOIN | 即使右表中没有匹配,也从左表返回所有的行 |
RIGHT JOIN | 即使左表中没有匹配,也从右表返回所有的行 |
FULL JOIN | 只要其中一个表中存在匹配,就返回行 |
UPDATE
-- 更新列
UPDATE Person SET FirstName = 'Fred' WHERE LastName = 'Wilson'
UPDATE Person SET Address = 'Zhongshan 23', City = 'Nanjing'
WHERE LastName = 'Wilson'
DELETE
-- 条件删除
DELETE FROM Person WHERE LastName = 'Wilson'
-- 删除所有行
DELETE FROM table_name
DELETE * FROM table_name
INSERT INTO
-- 插入行
INSERT INTO Persons VALUES ('Gates', 'Bill', 'Xuanwumen 10', 'Beijing')
-- 只给出指定列
INSERT INTO Persons (LastName, Address) VALUES ('Wilson', 'Champs-Elysees')
INDEX
-- 创建索引
CREATE INDEX PersonIndex ON Person (LastName)
-- 降序索引、多列索引
CREATE INDEX PersonIndex ON Person (LastName DESC, FirstName)
-- 删除索引
ALTER TABLE table_name DROP INDEX index_name
VIEW
-- 创建/更新视图
CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No
-- 选取 Products 表中所有单位价格高于平均单位价格的产品
CREATE VIEW [Products Above Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
-- 删除视图
DROP VIEW view_name
-- 查询视图
SELECT * FROM [Products Above Average Price]
CONSTRAIN
数据类型
数据类型 | 描述 |
---|---|
integer(size) int(size) smallint(size) tinyint(size) | 仅容纳整数。在括号内规定数字的最大位数。 |
decimal(size,d) numeric(size,d) | 容纳带有小数的数字。 “size” 规定数字的最大位数。”d” 规定小数点右侧的最大位数。 |
char(size) | 容纳固定长度的字符串(可容纳字母、数字以及特殊字符)。在括号中规定字符串的长度。 |
varchar(size) | 容纳可变长度的字符串(可容纳字母、数字以及特殊的字符)。在括号中规定字符串的最大长度。 |
date(yyyymmdd) | 容纳日期。 |
NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, DEFAULT
-- 匿名约束
CREATE TABLE Persons (
Id_P int NOT NULL AUTO_INCREMENT,
City varchar(255) DEFAULT 'Sandnes',
UNIQUE (Id_P),
PRIMARY KEY (Id_P),
CHECK (Id_P>0)
)
CREATE TABLE Orders (
Id_O int NOT NULL,
Id_P int,
OrderDate date DEFAULT GETDATE(),
PRIMARY KEY (Id_O),
FOREIGN KEY (Id_P) REFERENCES Persons(Id_P)
ON DELETE SET DEFAULT ON UPDATE CASCADE
)
-- 命名约束
CREATE TABLE Persons (
Id_P int NOT NULL,
CONSTRAINT uc_PersonID UNIQUE (Id_P,LastName),
CONSTRAINT pk_PersonID PRIMARY KEY (Id_P,LastName)
CONSTRAINT chk_Person CHECK (Id_P>0 AND City='Sandnes')
)
CREATE TABLE Orders (
Id_O int NOT NULL,
Id_P int,
PRIMARY KEY (Id_O),
CONSTRAINT fk_PerOrders FOREIGN KEY (Id_P) REFERENCES Persons(Id_P)
)
-- 添加 UNIQUE 约束
ALTER TABLE Persons ADD UNIQUE (Id_P)
ALTER TABLE Persons ADD CONSTRAINT uc_PersonID UNIQUE (Id_P,LastName)
-- 删除 UNIQUE 约束
ALTER TABLE Persons DROP INDEX uc_PersonID
-- 添加主键
ALTER TABLE Persons ADD PRIMARY KEY (Id_P)
ADD CONSTRAINT pk_PersonID PRIMARY KEY (Id_P,LastName)
-- 删除主键
ALTER TABLE Persons DROP PRIMARY KEY
-- 添加外键
ALTER TABLE Orders ADD FOREIGN KEY (Id_P) REFERENCES Persons(Id_P)
ALTER TABLE Orders ADD CONSTRAINT fk_PerOrders FOREIGN KEY (Id_P) REFERENCES Persons(Id_P)
-- 删除外键
ALTER TABLE Orders DROP FOREIGN KEY fk_PerOrders
-- 添加 CHECK 约束
ALTER TABLE Persons ADD CHECK (Id_P>0)
ALTER TABLE Persons ADD CONSTRAINT chk_Person CHECK (Id_P>0 AND City='Sandnes')
-- 删除 CHECK 约束
ALTER TABLE Persons DROP CHECK chk_Person
-- 添加DEFAULT约束
ALTER TABLE Persons ALTER City SET DEFAULT 'SANDNES'
-- 删除DEFAULT约束
ALTER TABLE Persons ALTER City DROP DEFAULT
-- 更改自增偏移
ALTER TABLE Persons AUTO_INCREMENT=100
Comments
请在后台配置评论类型和相关的值。