【EFCORE笔记】在远程查询数据

生成操作

DefaultIfEmpty、Empty、Range 和 Repeat，Iqueryable 只支持 DefaultIfEmpty 方法。

var blogs = _context.Blogs.Where(b => b.BlogId > 1).DefaultIfEmpty();

　　

var defaultBlog = new Blog { Name = "零度" };
var blogs = _context.Blogs.Where(b => b.BlogId > 1).DefaultIfEmpty(defaultBlog);

　　

 

相等比较

SequenceEqual

var blogs1 = _context.Blogs.Where(b => b.BlogId > 5);

var blogs2 = _context.Blogs.Where(b => b.BlogId < 2);

var result = blogs1.SequenceEqual(blogs2);

　　

　　

var blogs1 = _context.Blogs.Where(b => b.BlogId > 5).AsEnumerable();

var blogs2 = _context.Blogs.Where(b => b.BlogId < 2).AsEnumerable();

var result = blogs1.SequenceEqual(blogs2);

　　

 

串联运算

Concat

var blogs1 = _context.Blogs.Where(b => b.BlogId > 5); 

var blogs2 = _context.Blogs.Where(b => b.BlogId < 2);

var blogs3 = blogs1.Concat(blogs2);

　　

联接运算

Join 和 GroupJoin

var query = _context.Blogs.Join(_context.Posts,
        b => b.BlogId,
        p => p.BlogId,
        (b, p) => new { b.BlogId, BlogTitle = p.Title, p.Content }
        );

　　

var query = _context.Blogs.GroupJoin(_context.Posts,
        b => b.BlogId,
        p => p.BlogId,
        (b, ps) => new { b.BlogId, Posts=ps }
        );

　　

数据分组

GroupBy 和 ToLookup

var query = _context.Blogs.Where(b=>b.BlogId>2).GroupBy(b=>b.OwnerId);

　　

var query = _context.Blogs.Where(b=>b.BlogId>2).GroupBy(b=>b.Owner);

　　

var query1 = _context.Blogs.Where(b=>b.BlogId>2).ToLookup(b =>b.OwnerId); 

var query2 = _context.Blogs.Where(b => b.BlogId > 2).ToLookup(b => b.Owner);

　　

投影运算

Select 与 SelectMany

var query = _context.Blogs.Select(b =>
 new { BlogName = b.Name + "$", b.Url });

　　

var query = _context.Blogs
        .Select(b => 
new { b.BlogId, b.Name, Posts = _context.Posts.Where(p =>p.BlogId == b.BlogId) })
        .SelectMany(b => b.Posts);

　　

图解各种JOIN的区别

深入理解SQL中的Join机制

SQL夯实基础各种联接查询

数据分区

Skip、SkipWhile、Take 和 TakeWhile，SkipLast 和 TakeLast

var query = _context.Blogs.Where(b=>b.BlogId>2).Skip(2);

　　

分页实现：OFFSET-FETCH

var query = _context.Blogs.Where(b=>b.BlogId>2).Take(2);

　　

var query = _context.Blogs.Where(b => b.BlogId > 2).Skip(10).Take(10);

var query = _context.Blogs.SkipWhile(b => b.BlogId > 2);

 var query = _context.Blogs.TakeWhile(b => b.BlogId > 2); 

var query = _context.Blogs.SkipLast(2);

var query = _context.Blogs.TakeLast(2);

　　

数据排序

OrderBy、OrderByDescending、ThenBy、ThenByDescending 和 Reverse

var query = _context.Blogs.Where(b => b.BlogId > 2).OrderBy(b => b.OwnerId);

　　

var query = _context.Blogs
.Where(b => b.BlogId > 2).OrderBy(b => b.OwnerId).OrderByDescending(b=>b.BlogId);

var query = _context.Blogs
.Where(b => b.BlogId > 2).OrderBy(b => b.OwnerId).ThenByDescending(b 
=> b.BlogId);

　　

var query = _context.Blogs.Where(b => b.BlogId > 2).Reverse();

　　

var query = _context.Blogs.Where(b => b.BlogId > 2).AsEnumerable().Reverse();

　　

集合运算

Distinct、Except、Intersect 和 Union

var blogs = _context.Blogs.Where(b => b.BlogId > 5);
var query = blogs.Distinct();

　　

var blogs1 = _context.Blogs.Where(b => b.BlogId > 5); 

var blogs2 = _context.Blogs.Where(b => b.BlogId > 2);

var query = blogs1.Except(blogs2); 

var query = blogs1.Intersect(blogs2); 

var query = blogs1.Union(blogs2);

　　

 

 

限定符运算

All、Any 和 Contains

bool query = _context.Blogs.All(b => b.BlogId > 5);

　　

bool query = _context.Blogs.Any(b => b.BlogId > 5);

　　

var names = new string[] { "A", "B", "C" };

bool query = _context.Blogs.Any(b => names.Contains(b.Name));

　　

var blog = new Blog { BlogId = 1 };
bool query = _context.Blogs.Contains(blog);

　　

bool query = _context.Blogs.Where(b => b.BlogId > 2).Contains(blog);

　　

var query = _context.Blogs.Where(b => b.Name.Contains("ABC"));

var query = _context.Blogs.Where(b => b.Name.StartsWith("A"));

var query = _context.Blogs.Where(b => b.Name.EndsWith("ABC"));