1. 用Struct或Map接收数据

1.1 Find到Struct

定义一个小的结构体来接收表中的查询结果

db.Model(User{}).Find(&UserModels)

• 完整示例
  已有数据表

mysql> select * from users;
+----+----------+------+--------------------+
| id | name     | age  | email              |
+----+----------+------+--------------------+
|  1 | LiuBei   |   28 | liubei@xishu.com   |
|  2 | GuanYu   |   22 | guanyu@xishu.com   |
|  3 | ZhangFei |   20 | zhangfei@xishu.com |
+----+----------+------+--------------------+
3 rows in set (0.00 sec)

代码

package main

import (
    "database/sql"
    "fmt"
    "gorm.io/driver/mysql"
    "gorm.io/gorm"
    "time"
)

type User struct {
    ID int64
    Age int64
    Name string
    Email string
}

type userModel struct {
    ID int64
    Name string
}

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var UserModels  []userModel
    db.Model(User{}).Find(&UserModels)
    fmt.Println(UserModels)
}

func connect() (db *gorm.DB,sqlDB *sql.DB,err error) {
    dsn := "root:40010355@tcp(127.0.0.1:3306)/crow?charset=utf8&parseTime=True&loc=Local"
    db, err = gorm.Open(mysql.Open(dsn), &gorm.Config{})
    sqlDB,_ = db.DB()
    if err != nil {
        fmt.Printf(err.Error())
        defer sqlDB.Close()
    }else {
        fmt.Printf("OK\n")
        sqlDB.SetMaxIdleConns(10)
        sqlDB.SetMaxOpenConns(100)
        sqlDB.SetConnMaxLifetime(time.Hour)
    }
    return
}

结果输出

OK
[{1 LiuBei} {2 GuanYu} {3 ZhangFei}]

1.2 Find到Map

定义一个Map来接收数据

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()
    
    var users  map[string]interface{}
    db.Table("users").Find(&users)
    fmt.Println(users)
}

2. 子查询

db.Where("age > (?)", db.Table("users").Select("AVG(age)")).Find(&users)
// SELECT * FROM "users" WHERE age > (SELECT AVG(age) FROM "users");

• 示例

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var users []User
    db.Where("age > (?)", db.Table("users").Select("AVG(age)")).Find(&users)
    fmt.Println(users)
}

• 查询结果

OK
[{1 28 LiuBei liubei@xishu.com}]

3. Group/Having

3.1 Group

求和、平均值等情况的分组

db.Table("users").Select("AVG(age) as avg_age","company").Group("company").Find(&users)

完整示例

求下表中各公司的平均年龄

• 数据表

mysql> select * from users;
+----+----------+------+--------------------+---------+
| id | name     | age  | email              | company |
+----+----------+------+--------------------+---------+
|  1 | LiuBei   |   28 | liubei@xishu.com   | shu     |
|  2 | GuanYu   |   22 | guanyu@xishu.com   | shu     |
|  3 | ZhangFei |   20 | zhangfei@xishu.com | shu     |
|  4 | SunQuan  |   22 | sunquan@dongwu.com | wu      |
|  5 | ZhouYu   |   15 | zhouyu@dongwu.com  | wu      |
+----+----------+------+--------------------+---------+
5 rows in set (0.00 sec)

• 代码

package main

import (
    "database/sql"
    "fmt"
    "gorm.io/driver/mysql"
    "gorm.io/gorm"
    "time"
)

type User struct {
    ID int64
    Age int64
    Name string
    Email string
    Company string
}

type groupBy struct {
    AvgAge float64
    Company string

}


func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var users []groupBy

    db.Table("users").Select("AVG(age) as avg_age","company").Group("company").Find(&users)
    fmt.Println(users)
}

func connect() (db *gorm.DB,sqlDB *sql.DB,err error) {
    dsn := "root:40010355@tcp(127.0.0.1:3306)/crow?charset=utf8&parseTime=True&loc=Local"
    db, err = gorm.Open(mysql.Open(dsn), &gorm.Config{})
    sqlDB,_ = db.DB()
    if err != nil {
        fmt.Printf(err.Error())
        defer sqlDB.Close()
    }else {
        fmt.Printf("OK\n")
        sqlDB.SetMaxIdleConns(10)
        sqlDB.SetMaxOpenConns(100)
        sqlDB.SetConnMaxLifetime(time.Hour)
    }
    return
}

• 输出结果

OK
[{23.3333 shu} {18.5 wu}]

3.2 Having

对Group的结果进行过滤。上例中得到的结果，用Having得到平均值大于20的结果。

db.Table("users").Select("AVG(age) as avg_age","company").Group("company").Having("avg_age > ?",20).Find(&users)

4. 变量

4.1 使用sql.Named定义

db.Where("age > @age AND company = @company", sql.Named("company", "shu"),sql.Named("age", 20)).Find(&users)

4.2 使用map定义变量

db.Where("age > @age AND company = @company",map[string]interface{}{"company": "shu","age":20}).First(&users)

• 示例

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var users []User

    checkUser := map[string]interface{}{
        "company":"shu",
        "age":20,
    }

    db.Where("age > @age AND company = @company",checkUser).First(&users)
    fmt.Println(users)
}

5. 用Rows() 迭代

• 使用
  用Rows() 记录数据 ----> rows.Next()逐行读出-----> 用ScanRows赋值给结构体

• 示例

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    rows, _ := db.Model(&User{}).Where("company = ?", "shu").Rows()
    defer rows.Close()

    for rows.Next() {
        var user User
        // ScanRows 方法用于将一行记录扫描至结构体
        db.ScanRows(rows, &user)
        fmt.Println(user)
    }
}

6. 查钩子

写一个查询钩子，当查询结果字段Company为空时，替换为"群"

func (u *User) AfterFind(tx *gorm.DB) (err error) {
    if u.Company == "" {
        u.Company = "qun"
    }
    return
}

7. Pluck（单列查询）

• 作用
  查询单列数据，并用一个切片接收
• 样例

db.Model(&User{}).Pluck("name", &names)

• 代码

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()
    
    var names []string
    db.Model(&User{}).Pluck("name", &names)
    fmt.Println(names)
}

• 输出

[LiuBei GuanYu ZhangFei YuJi ZhaoYun]

8. Scopes（调用查询函数）

8.1 使用

• 作用
  我们可以使用Scopes调用事先创建好的函数。
• 样例

db.Scopes(函数1,函数2).Find(&users)

8.2 示例

• 创建函数

创建一个函数用来查询年龄大约20岁的用户数据

func ageGT20db (db *gorm.DB) *gorm.DB {
    return db.Where("age > ?",20)
}

• 调用函数

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var users []User

    db.Scopes(ageGT20db).Find(&users)
    fmt.Println(users)
}

9. Count

• 作用
  统计查到了多少条数据

• 代码

func main() {
    db,sqlDB,_ := connect()
    defer sqlDB.Close()

    var count int64
    var users []User

    db.Find(&users).Count(&count)
    fmt.Println(count)
}