SQL常见知识点汇总

俗话说：地基不牢，地动山摇。SQL常见知识点还是很重要的，掌握了，操作数据库是错错有余。

1. 数据库基本操作

创建数据库

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-- 创建数据库
CREATE DATABASE EmployeeDB;
GO

-- 使用数据库
USE EmployeeDB;
GO

创建表

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-- 创建员工表
CREATETABLE Employees (
    EmployeeID INTIDENTITY(1,1) PRIMARY KEY,
    FirstName NVARCHAR(50) NOTNULL,
    LastName NVARCHAR(50) NOTNULL,
    Email NVARCHAR(100) UNIQUE,
    HireDate DATEDEFAULTGETDATE(),
    Salary DECIMAL(10,2) CHECK (Salary > 0),
    DepartmentID INT
);

-- 创建部门表
CREATETABLE Departments (
    DepartmentID INTIDENTITY(1,1) PRIMARY KEY,
    DepartmentName NVARCHAR(100) NOTNULL,
    ManagerID INT
);

2. 数据作语言（DML）

插入数据

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-- 插入部门数据
INSERTINTO Departments (DepartmentName) 
VALUES
    ('技术部'),
    ('销售部'),
    ('人力资源部');

-- 插入员工数据
INSERTINTO Employees (FirstName, LastName, Email, Salary, DepartmentID)
VALUES
    ('张三', '张', 'zhangsan@company.com', 8000.00, 1),
    ('李四', '李', 'lisi@company.com', 7500.00, 1),
    ('王五', '王', 'wangwu@company.com', 9000.00, 2);

更新数据

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-- 更新员工薪资
UPDATE Employees 
SET Salary = Salary * 1.1 
WHERE DepartmentID = 1;

-- 使用JOIN更新
UPDATE e
SET e.Salary = e.Salary * 1.05
FROM Employees e
INNER JOIN Departments d ON e.DepartmentID = d.DepartmentID
WHERE d.DepartmentName = '技术部';

删除数据

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-- 删除特定记录
DELETE FROM Employees 
WHERE EmployeeID = 3;

-- 使用事务删除
BEGIN TRANSACTION;
DELETE FROM Employees 
WHERE HireDate < '2020-01-01';
COMMIT TRANSACTION;

3. 数据查询

基础查询

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-- 选择所有列
SELECT * FROM Employees;

-- 选择特定列
SELECT FirstName, LastName, Salary 
FROM Employees;

-- 使用别名
SELECT
    FirstName AS'名字',
    LastName AS'姓氏',
    Salary AS'薪资'
FROM Employees;

WHERE条件查询

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-- 基本条件查询
SELECT * FROM Employees 
WHERE Salary > 8000;

-- 多条件查询
SELECT * FROM Employees 
WHERE Salary BETWEEN7000AND9000
    AND DepartmentID = 1;

-- LIKE模糊查询
SELECT * FROM Employees 
WHERE LastName LIKE'张%';

-- IN查询
SELECT * FROM Employees 
WHERE DepartmentID IN (1, 2);

聚合函数

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-- 常用聚合函数
SELECT 
    COUNT(*) AS '员工总数',
    AVG(Salary) AS '平均薪资',
    MAX(Salary) AS '最高薪资',
    MIN(Salary) AS '最低薪资',
    SUM(Salary) AS '薪资总额'
FROM Employees;

4. 表连接查询

INNER JOIN

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-- 内连接查询员工和部门信息
SELECT 
    e.FirstName,
    e.LastName,
    e.Salary,
    d.DepartmentName
FROM Employees e
INNER JOIN Departments d ON e.DepartmentID = d.DepartmentID;

LEFT/RIGHT JOIN

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-- 左外连接：显示所有员工，即使没有部门
SELECT
    e.FirstName,
    e.LastName,
    d.DepartmentName
FROM Employees e
LEFTJOIN Departments d ON e.DepartmentID = d.DepartmentID;

-- 右外连接：显示所有部门，即使没有员工
SELECT
    e.FirstName,
    e.LastName,
    d.DepartmentName
FROM Employees e
RIGHTJOIN Departments d ON e.DepartmentID = d.DepartmentID;

5. 高级查询技巧

分组查询

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-- 按部门分组统计
SELECT 
    d.DepartmentName,
    COUNT(e.EmployeeID) AS '员工数量',
    AVG(e.Salary) AS '平均薪资'
FROM Departments d
LEFT JOIN Employees e ON d.DepartmentID = e.DepartmentID
GROUP BY d.DepartmentID, d.DepartmentName
HAVING COUNT(e.EmployeeID) > 0;

子查询

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-- 使用子查询查找薪资高于平均值的员工
SELECT
    FirstName,
    LastName,
    Salary
FROM Employees
WHERE Salary > (SELECTAVG(Salary) FROM Employees);

-- 使用IN的子查询
SELECT
    FirstName,
    LastName
FROM Employees
WHERE DepartmentID IN (
    SELECT DepartmentID 
    FROM Departments 
    WHERE DepartmentName LIKE'%技术%'
);

6. 视图的创建与使用

创建视图

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-- 创建员工信息视图
CREATE VIEW vw_EmployeeDetails AS
SELECT 
    e.EmployeeID,
    e.FirstName + ' ' + e.LastName AS FullName,
    e.Email,
    e.Salary,
    d.DepartmentName,
    e.HireDate
FROM Employees e
INNER JOIN Departments d ON e.DepartmentID = d.DepartmentID;

使用视图

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-- 查询视图
SELECT * FROM vw_EmployeeDetails 
WHERE Salary > 8000;

-- 更新视图（有限制条件）
UPDATE vw_EmployeeDetails 
SET Salary = 8500 
WHERE EmployeeID = 1;

7. 存储过程

创建存储过程

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-- 创建获取员工信息的存储过程
CREATEPROCEDURE sp_GetEmployeesByDepartment
    @DepartmentID INT = NULL,
    @MinSalary DECIMAL(10,2) = 0
AS
BEGIN
    SELECT
        FirstName,
        LastName,
        Salary,
        HireDate
    FROM Employees
    WHERE (@DepartmentID ISNULLOR DepartmentID = @DepartmentID)
        AND Salary >= @MinSalary
    ORDERBY Salary DESC;
END;

执行存储过程

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-- 执行存储过程
EXEC sp_GetEmployeesByDepartment @DepartmentID = 1, @MinSalary = 7000;

-- 使用默认参数
EXEC sp_GetEmployeesByDepartment;

8. 索引优化

创建索引

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-- 创建单列索引
CREATEINDEX IX_Employees_DepartmentID 
ON Employees(DepartmentID);

-- 创建复合索引
CREATEINDEX IX_Employees_Name 
ON Employees(LastName, FirstName);

-- 创建唯一索引
CREATEUNIQUEINDEX IX_Employees_Email 
ON Employees(Email);

-- 查看索引信息
SELECT
    nameAS IndexName,
    type_desc ASIndexType
FROM sys.indexes 
WHERE object_id = OBJECT_ID('Employees');

9. 事务处理

事务基本使用

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-- 简单事务示例
BEGINTRANSACTION;

BEGIN TRY
    -- 更新员工薪资
    UPDATE Employees 
    SET Salary = Salary * 1.1
    WHERE DepartmentID = 1;
    
    -- 记录日志（假设有Log表）
    INSERTINTO SalaryLog (EmployeeID, OldSalary, NewSalary, ChangeDate)
    SELECT
        EmployeeID, 
        Salary / 1.1, 
        Salary, 
        GETDATE()
    FROM Employees 
    WHERE DepartmentID = 1;
    
    COMMITTRANSACTION;
    PRINT '事务执行成功';
END TRY
BEGIN CATCH
    ROLLBACKTRANSACTION;
    PRINT '事务执行失败: ' + ERROR_MESSAGE();
END CATCH;

10. 常用系统函数

字符串函数

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-- 字符串处理示例
SELECT 
    FirstName,
    LastName,
    UPPER(FirstName) AS UpperFirstName,
    LOWER(LastName) AS LowerLastName,
    LEN(FirstName) AS NameLength,
    LEFT(FirstName, 1) AS FirstInitial,
    REVERSE(FirstName) AS ReversedName
FROM Employees;

日期函数

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-- 日期处理示例
SELECT 
    FirstName,
    HireDate,
    GETDATE() AS CurrentDate,
    DATEDIFF(YEAR, HireDate, GETDATE()) AS YearsOfWork,
    DATEADD(MONTH, 6, HireDate) AS ProbationEndDate,
    DATENAME(MONTH, HireDate) AS HireMonth
FROM Employees;

11. 窗口函数

排名函数

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-- 使用窗口函数进行排名
SELECT
    FirstName,
    LastName,
    Salary,
    DepartmentID,
    ROW_NUMBER() OVER (PARTITIONBY DepartmentID ORDERBY Salary DESC) AS DeptSalaryRank,
    RANK() OVER (ORDERBY Salary DESC) AS OverallSalaryRank,
    DENSE_RANK() OVER (ORDERBY Salary DESC) AS DenseSalaryRank,
    NTILE(4) OVER (ORDERBY Salary DESC) AS SalaryQuartile
FROM Employees;

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