Multi-table query, commonly used have a league table LeftJoin/InnerJoin/RightJoin, these three methods has introduced in the article.

In addition to a join table, there are subqueries Where Exists and Select subtables:

static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
    .UseConnectionString(FreeSql.DataType.MySql, "Data Source = 127.0.0.1; Port=3306; User ID=root; Password=root; Initial Catalog=cccddd; Charset=utf8; SslMode=none; Max pool size=10")
    .Build(); // Be sure to define the Singleton Singleton pattern

class Topic {
    [Column(IsIdentity = true)]
    public int Id { get; set; }
    public string Title { get; set; }
    public int Clicks { get; set; }
    public DateTime CreateTime { get; set; }

    public int CategoryId { get; set; }
    public Category Category { get; set; }}class Category {
    [Column(IsIdentity = true)]
    public int Id { get; set; }
    public string Name { get; set; }

    public int ParentId { get; set; }
    public CategoryType Parent { get; set; }
    public List<Topic> Topics { get; set; }}class CategoryType {
    public int Id { get; set; }
    public string Name { get; set; }}Copy the code

1. The subtable Exists

sql = fsql.Select<Topic>()
  .Where(a => fsql.Select<Topic>().As("b").Where(b => b.Id == a.Id).Any())
  .ToList();
//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`
//FROM `Topic` a
//WHERE (exists(SELECT 1
// FROM `Topic` b
// WHERE (b.`Id` = a.`Id`)
/ / limit 0, 1))
Copy the code

Tip: Since the entity class of the subquery is the same As that of the upper layer, use As(“b”) to specify the alias to distinguish it

2, child table In

fsql.Select<Topic>()
  .Where(a => fsql.Select<Topic>().As("b").ToList(b => b.Id).Contains(a.Id))
  .ToList();
//SELECT a.`Id`, a.`Title`, a.`Clicks`, a.`CreateTime`, a.`CategoryId`
//FROM `Topic` a
//WHERE (((a.`Id`) in (SELECT b.`Id`
// FROM `Topic` b)))
Copy the code

3, the child table First/Count/Sum/Max/Min/Avg

fsql.Select<Category>().ToList(a => new 
{
  all = a,
  first = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).First(b => b.Id),
  count = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Count(),
  sum = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Sum(b => b.Clicks),
  max = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Max(b => b.Clicks),
  min = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Min(b => b.Clicks),
  avg = fsql.Select<Topic>().Where(b => b.CategoryId == a.Id).Avg(b => b.Clicks)
});
Copy the code

4, AsSelect

fsql.Select<Category>()
  .Where(a => a.Topics.AsSelect().Any(b => b.Title.Contains("xx")))
  .ToList();
Copy the code

The effect is equivalent to:

fsql.Select<Category>()
  .Where(a => fsql.Select<Topic>().Any(b => b.Title.Contains("xx")))
  .ToList();
Copy the code

Quickly convert collection properties to ISelect for subquery operations.

Series article navigation

  • (1) Introduction

  • (2) Automatic migration of entities

  • (3) Entity characteristics

  • (4) Solid features of Fluent Api

  • (5) Insert data

  • (6) Batch insert data

  • (7) Ignore columns when inserting data

  • (8) Specify columns when inserting data

  • (9) Delete data

  • (x) Update data

  • (11) Update data Where

  • (12) Specify columns when updating data

  • (13) Ignore columns when updating data

  • (14) Batch update data

  • (15) Query data

  • (16) paging query

  • (17) joint table query

  • (18) Navigation attributes

  • (19) multi-table query

  • (20) query where ecascade

  • (21) Query returned data

  • (22) Dto mapping query

  • (23) Grouping and aggregation

  • (24) Introduction To Linq To Sql syntax

  • (25) delayed loading

  • Include, IncludeMany, Dto, ToList

  • (27) the SQL statement has been written, and entity class mapping for the second query

  • (28) Business

  • Lambda expression

  • (30) Reading and writing separation

  • (31) Zoning table

  • (32) Aop

  • CodeFirst type mapping

  • (34) CodeFirst migration instructions

  • CodeFirst custom features

The resources
Beginner’s Guide | “Select” | “Update” | “Insert” | “Delete”
Expression function | “CodeFirst” | “DbFirst” | “The BaseEntity”
“Repository” | “The UnitOfWork” | The Filter | Optimism Lock | “The DbContext”
Unread | Partition table | “The tenants” | The AOP | Black Tech | Update log