How to use SQL for joint query in Golang

In Golang, the SQL joint query statement uses multiple SQL query statements to combine into a query result set. This method can be used to jointly query two or more tables and improve query efficiency. This article will introduce how to use Golang to perform union queries in SQL.

What is SQL union query?

SQL Union Query refers to the process of combining the result sets of two or more SELECT statements into one result set. The number of columns, column order, and column types queried in the aggregate query must be exactly the same. Union query is a relatively efficient query method that improves query efficiency by combining multiple query statements instead of multiple queries.

In SQL, union queries are implemented using the UNION operator. The UNION operator combines two result sets and removes duplicate rows. If you want to keep duplicate rows, you can use UNION ALL operator.

Union query in Golang

When using SQL in Golang to perform a joint query, we need to first connect to the database and use SQL statements to execute the joint query. In this example, we will use the Go language ORM library GORM to connect to the MySQL database and use a union query to retrieve data from the two tables.

To use GORM to implement SQL union queries, we need to call the Model function to create two different model objects and use the Select function to combine them together. The Select function selects the columns to be retrieved and saves the results in a new model object.

The following is an example of using Golang and GORM libraries to implement SQL union queries:

package main

import (
    "fmt"
    "time"

    "gorm.io/driver/mysql"
    "gorm.io/gorm"
)

type User struct {
    ID        uint `gorm:"primary_key"`
    Name      string
    Age       int
    Email     string
    CreatedAt time.Time
}

type Order struct {
    ID        uint `gorm:"primary_key"`
    UserID    uint
    Amount    float64
    CreatedAt time.Time
}

func main() {
    dsn := "user:password@tcp(localhost:3306)/database_name?charset=utf8mb4&parseTime=True&loc=Local"
    db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
    if err != nil {
        panic("failed to connect database")
    }

    var users []User
    db.Model(&User{}).Select("users.name, orders.amount").Joins("inner join orders on orders.user_id = users.id").Find(&users)

    fmt.Println(users)
}

In the above code example, we first define two model objects: User and Order. We then connected to a MySQL database named "database_name" and created a new model object named "users" using GORM's Model function. In the Select function, we select the columns that need to be retrieved and define an inner join query in the Joins function. Finally, we use the Find function to save the query results in a slice called "users" and print them to the terminal.

Conclusion

In Golang, we can use the ORM library GORM to connect to the MySQL database and perform SQL union queries. Use joint queries to combine data from multiple tables and improve query efficiency. Using the above sample code, implementing SQL union queries in your applications will become easier and more efficient.

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