golang joint table query

In golang development, we often need to use databases for data operations, and in actual business we often need to perform joint table queries on multiple tables. This article will introduce how to use golang to perform multi-table joint queries.

1. Install golang’s ORM tool

In golang, we often use ORM tools to perform database operations. ORM (Object Relational Mapping), which is Object Relational Mapping, converts tables in relational databases into object-oriented forms, making database operations more flexible and convenient. Commonly used golang ORM tools include GORM, XORM, Beego ORM, etc. Here we take using GORM as an example.

You can install GORM through the following command:

go get -u github.com/jinzhu/gorm

2. Define the data model

Before performing joint table query, you need to define the data model. The data model converts relational database tables into structures in golang to facilitate our data operations. For example, we need to perform a joint table query of two tables, one is the user table user, and the other is the order table order. User and order can be defined as the following structures respectively:

type User struct {
    Id   int
    Name string
}

type Order struct {
    Id         int
    UserId     int
    OrderName  string
    CreateTime time.Time
}

Among them, the User structure It includes two fields, namely Id and Name, which correspond to the id and name fields in the user table; the Order structure includes four fields, namely Id, UserId, OrderName and CreateTime, which correspond to the id, user_id, order_name and create_time fields.

3. Define the association

When performing a joint table query, you need to define the association between the two tables. GORM provides four types of relationships: One-To-One, One-To-Many, Many-To-One, Many-To-Many ).

For example, we need to perform a many-to-one association between User and Order, that is, one user can correspond to multiple orders, and one order can only correspond to one user. You can add an Orders field to the User structure to represent all the orders corresponding to a user, as follows:

type User struct {
    Id     int
    Name   string
    Orders []Order `gorm:"ForeignKey:UserId"`
}

In the Order structure, you need to add a User field to represent the users corresponding to an order. As follows:

type Order struct {
    Id         int
    UserId     int
    User       User
    OrderName  string
    CreateTime time.Time
}

Among them, "ForeignKey:UserId" in the Orders field indicates that the user_id field in the orders table is associated with the id field in the users table and is used as a foreign key in the users table; the User field User in indicates that this field is associated with the Orders field in the User structure.

4. Conducting joint query

When performing joint query of multiple tables through golang, you can use the Preload method of GORM. The Preload method can query all relevant data based on the association relationship. For example, if we need to query all orders and include their corresponding user information, we can use the following code:

var orders []Order 

db.Preload("User").Find(&orders)

fmt.Println(orders[0].User.Name)  // 输出第一条数据的用户名

Among them, Preload("User") means using the User field for association when querying, and Find(&orders) means Query all orders and store them in the orders variable. orders[0].User.Name represents the user name of the user corresponding to the first piece of data output.

If we need to query all users and their corresponding orders, we can use the following code:

var users []User

db.Preload("Orders").Find(&users)

fmt.Println(users[0].Orders[0].OrderName)  // 输出第一个用户的第一个订单名

Among them, Preload("Orders") means using the Orders field for association when querying, Find(&users ) means to query all users and store them in the users variable. users[0].Orders[0].OrderName means outputting the first order name of the first user.

Summary

Using golang to perform multi-table joint queries, we can implement it by defining the data model, defining the relationship, and using GORM's Preload method. Good data models and relationships can facilitate our data operations and queries. GORM's Preload method allows us to easily perform multi-table joint queries, reducing the amount of code and improving code efficiency.

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