LINQ 备忘单 - SQL

LINQ（Language Integrated Query）是一种用于 .NET 平台的集成查询语言，它可以在.NET 编程语言（如 C# 和 VB.NET）中方便地执行数据查询。通过 LINQ 可以将查询与.NET 程序代码融为一体，从而实现对数据的强大操作。本备忘单提供一些常用的 LINQ 查询代码，以及对应的 SQL 语句。

基本查询

查询语法

var result = from p in Products
             where p.Price > 100
             select p;

SELECT * FROM Products WHERE Price > 100

方法语法

var result = Products.Where(p => p.Price > 100);

SELECT * FROM Products WHERE Price > 100

投影查询

查询语法

var result = from p in Products
             where p.Price > 100
             select new { p.Name, p.Price };

SELECT Name, Price FROM Products WHERE Price > 100

方法语法

var result = Products.Where(p => p.Price > 100)
                     .Select(p => new { p.Name, p.Price });

SELECT Name, Price FROM Products WHERE Price > 100

排序查询

查询语法

var result = from p in Products
             orderby p.Price descending
             select p;

SELECT * FROM Products ORDER BY Price DESC

方法语法

var result = Products.OrderByDescending(p => p.Price);

SELECT * FROM Products ORDER BY Price DESC

分组查询

查询语法

var result = from p in Products
             group p by p.Category into g
             select new { Category = g.Key, Count = g.Count() };

SELECT Category, COUNT(*) AS Count FROM Products GROUP BY Category

方法语法

var result = Products.GroupBy(p => p.Category)
                     .Select(g => new { Category = g.Key, Count = g.Count() });

SELECT Category, COUNT(*) AS Count FROM Products GROUP BY Category

连接查询

查询语法

var result = from p in Products
             join s in Suppliers on p.SupplierID equals s.ID
             select new { ProductName = p.Name, SupplierName = s.Name };

SELECT p.Name AS ProductName, s.Name AS SupplierName 
FROM Products p JOIN Suppliers s ON p.SupplierID = s.ID

方法语法

var result = Products.Join(Suppliers, 
                           p => p.SupplierID, 
                           s => s.ID, 
                           (p, s) => new { ProductName = p.Name, SupplierName = s.Name });

SELECT p.Name AS ProductName, s.Name AS SupplierName 
FROM Products p JOIN Suppliers s ON p.SupplierID = s.ID

嵌套查询

查询语法

var result = from c in Categories
             select new { CategoryName = c.Name, 
                          Products = from p in Products
                                     where p.CategoryID == c.ID
                                     select p };

SELECT c.Name AS CategoryName, 
       (SELECT * FROM Products WHERE CategoryID = c.ID) AS Products
FROM Categories c

方法语法

var result = Categories.Select(c => new 
                    { 
                        CategoryName = c.Name, 
                        Products = Products.Where(p => p.CategoryID == c.ID) 
                    });

SELECT c.Name AS CategoryName, 
       (SELECT * FROM Products WHERE CategoryID = c.ID) AS Products
FROM Categories c

这些都是常见的 LINQ 查询技巧，可以提高编写 SQL 代码的效率。可以根据需要调整查询语法以实现更强大的操作。