How to use golang to implement cross-table data query

When using golang to develop web applications, you often encounter situations where you need to query data across tables. This article will introduce how to use golang to query data across tables.

1. Compound query statement

In database query, you can use compound query statement to query data across tables. Compound query statements can be implemented through joint queries and nested queries. In golang, you can use the Query and QueryRow functions in the sql package to implement compound query statements.

The following is a sample code that uses compound query statements to query data across tables:

func GetUserInfoByName(name string) ([]*User, error) {
    db, err := sql.Open("mysql", "root:password@tcp(127.0.0.1:3306)/test")
    if err != nil {
        return nil, err
    }

    rows, err := db.Query("SELECT u.id, u.name, u.age, c.city FROM user u LEFT JOIN city c ON u.city_id = c.id WHERE u.name=?", name)
    if err != nil {
        return nil, err
    }
    defer rows.Close()

    users := make([]*User, 0)

    for rows.Next() {
        u := new(User)
        err := rows.Scan(&u.ID, &u.Name, &u.Age, &u.City)
        if err != nil {
            return nil, err
        }
        users = append(users, u)
    }

    if err := rows.Err(); err != nil {
        return nil, err
    }

    return users, nil
}

In the above sample code, we join the user table and city table through a left join, and query The user's ID, name, age and city. The query results are mapped to the User structure through the Scan function, and finally the query results are returned.

2. Use the ORM framework

In addition to using compound query statements, we can also use the ORM framework to query data across tables. The ORM framework maps database tables to Golang structures, allowing us to operate the database in an object-oriented manner.

In golang, using the ORM framework requires installing the GORM library. The following is a sample code for using GORM to query data across tables:

func GetUserInfoByName(name string) ([]*User, error) {
    db, err := gorm.Open("mysql", "root:password@tcp(127.0.0.1:3306)/test?charset=utf8mb4&parseTime=True&loc=Local")
    if err != nil {
        return nil, err
    }
    defer db.Close()

    var users []*User
    err = db.Table("user").Select("user.id, user.name, user.age, city.city").Joins("LEFT JOIN city city ON user.city_id = city.id").Where("user.name = ?", name).Scan(&users).Error
    if err != nil {
        return nil, err
    }

    return users, nil
}

In the above sample code, we use the ORM framework GORM to query data across tables. Use the Table function to select the table to be queried, use the Select function to specify the fields to be queried, use the Joins function to associate another table, use the Where function to specify the query conditions, and finally use the Scan function to map the query results to the User structure and return the query results. .

Summary:

This article introduces two methods of using golang to query data across tables: compound query statements and ORM framework. In actual development, developers can choose a suitable method according to their own needs.

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