Gorm框架学习---CRUD接口之查询

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Gorm框架学习---CRUD接口之查询

环境搭建

package main

import (
	"gorm.io/driver/mysql"
	"gorm.io/gorm"
	"gorm.io/gorm/logger"
)

const MYSQL_ADDR="user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local"


var DB *gorm.DB

type User struct 
	gorm.Model
	Age  int
	Name string


func main() 
	DB = openDB()
	//创建表
	DB.AutoMigrate(&User)
	//插入一条记录
	DB.Create(&User
		Name: "dhy",
		Age:  18,
	)


func openDB() *gorm.DB 
	//创建数据库连接
	db, err := gorm.Open(mysql.Open(MYSQL_ADDR), 
    //开启info级别的日志输出
    &gorm.ConfigLogger: logger.Default.LogMode(logger.Info))
	if err != nil 
		panic("failed to connect database")
	
	return db

检索单个对象

GORM 提供了 First、Take、Last 方法，以便从数据库中检索单个对象。当查询数据库时它添加了 LIMIT 1 条件，且没有找到记录时，它会返回 ErrRecordNotFound 错误

// 获取第一条记录（主键升序）
db.First(&user)
// SELECT * FROM users ORDER BY id LIMIT 1;

// 获取一条记录，没有指定排序字段
db.Take(&user)
// SELECT * FROM users LIMIT 1;

// 获取最后一条记录（主键降序）
db.Last(&user)
// SELECT * FROM users ORDER BY id DESC LIMIT 1;

result := db.First(&user)
result.RowsAffected // 返回找到的记录数
result.Error        // returns error or nil

// 检查 ErrRecordNotFound 错误
errors.Is(result.Error, gorm.ErrRecordNotFound)

如果你想避免ErrRecordNotFound错误，你可以使用Find，比如db.Limit(1).Find(&user)，Find方法可以接受struct和slice的数据。

First 和 Last 会根据主键排序，分别查询第一条和最后一条记录。 只有在目标 struct 是指针或者通过 db.Model() 指定 model 时，该方法才有效。 此外，如果相关 model 没有定义主键，那么将按 model 的第一个字段进行排序。 例如:

var user User
var users []User

// works because destination struct is passed in
db.First(&user)
// SELECT * FROM `users` ORDER BY `users`.`id` LIMIT 1

// works because model is specified using `db.Model()`
result := map[string]interface
db.Model(&User).First(&result)
// SELECT * FROM `users` ORDER BY `users`.`id` LIMIT 1

// doesn't work
result := map[string]interface
db.Table("users").First(&result)

// works with Take
result := map[string]interface
db.Table("users").Take(&result)

// no primary key defined, results will be ordered by first field (i.e., `Code`)
type Language struct 
  Code string
  Name string

db.First(&Language)
// SELECT * FROM `languages` ORDER BY `languages`.`code` LIMIT 1

用主键检索

如果主键是数字类型，您可以使用 内联条件 来检索对象。 传入字符串参数时，需要特别注意 SQL 注入问题，查看 安全 获取详情.

db.First(&user, 10)
// SELECT * FROM users WHERE id = 10;

db.First(&user, "10")
// SELECT * FROM users WHERE id = 10;

db.Find(&users, []int1,2,3)
// SELECT * FROM users WHERE id IN (1,2,3);

如果主键是字符串（例如像 uuid），查询将被写成这样：

db.First(&user, "id = ?", "1b74413f-f3b8-409f-ac47-e8c062e3472a")
// SELECT * FROM users WHERE id = "1b74413f-f3b8-409f-ac47-e8c062e3472a";

当目标结构体存在名为ID的属性时，会将该属性作为主键去构建查询条件，例如:

var user = UserID: 10
db.First(&user)
// SELECT * FROM users WHERE id = 10;

var result User
db.Model(UserID: 10).First(&result)
// SELECT * FROM users WHERE id = 10;

检索全部对象

// Get all records
result := db.Find(&users)
// SELECT * FROM users;

result.RowsAffected // returns found records count, equals `len(users)`
result.Error        // returns error

条件

String 条件

// Get first matched record
db.Where("name = ?", "jinzhu").First(&user)
// SELECT * FROM users WHERE name = 'jinzhu' ORDER BY id LIMIT 1;

// Get all matched records
db.Where("name <> ?", "jinzhu").Find(&users)
// SELECT * FROM users WHERE name <> 'jinzhu';

// IN
db.Where("name IN ?", []string"jinzhu", "jinzhu 2").Find(&users)
// SELECT * FROM users WHERE name IN ('jinzhu','jinzhu 2');

// LIKE
db.Where("name LIKE ?", "%jin%").Find(&users)
// SELECT * FROM users WHERE name LIKE '%jin%';

// AND
db.Where("name = ? AND age >= ?", "jinzhu", "22").Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' AND age >= 22;

// Time
db.Where("updated_at > ?", lastWeek).Find(&users)
// SELECT * FROM users WHERE updated_at > '2000-01-01 00:00:00';

// BETWEEN
db.Where("created_at BETWEEN ? AND ?", lastWeek, today).Find(&users)
// SELECT * FROM users WHERE created_at BETWEEN '2000-01-01 00:00:00' AND '2000-01-08 00:00:00';

Struct & Map 条件

// Struct
db.Where(&UserName: "jinzhu", Age: 20).First(&user)
// SELECT * FROM users WHERE name = "jinzhu" AND age = 20 ORDER BY id LIMIT 1;

// Map
db.Where(map[string]interface"name": "jinzhu", "age": 20).Find(&users)
// SELECT * FROM users WHERE name = "jinzhu" AND age = 20;

// Slice of primary keys
db.Where([]int6420, 21, 22).Find(&users)
// SELECT * FROM users WHERE id IN (20, 21, 22);

注意： 使用 struct 查询时，GORM 只会查询非零字段，这意味着如果您的字段的值为 0、‘’、false 或其他零值，则不会用于构建查询条件，例如：

db.Where(&UserName: "jinzhu", Age: 0).Find(&users)
// SELECT * FROM users WHERE name = "jinzhu";

要在查询条件中包含零值，您可以使用map，它将包含所有键值作为查询条件，例如：

db.Where(map[string]interface"Name": "jinzhu", "Age": 0).Find(&users)
// SELECT * FROM users WHERE name = "jinzhu" AND age = 0;

指定结构体查询字段

上面是默认使用结构体全部字段进行查询，我们还可以指定结构体中某几个字段参与查询：

db.Where(&UserName: "jinzhu", "name", "Age").Find(&users)
// SELECT * FROM users WHERE name = "jinzhu" AND age = 0;

db.Where(&UserName: "jinzhu", "Age").Find(&users)
// SELECT * FROM users WHERE age = 0;

内联条件

查询条件可以以与 Where 类似的方式内联到 First 和 Find 等方法中。

// Get by primary key if it were a non-integer type
db.First(&user, "id = ?", "string_primary_key")
// SELECT * FROM users WHERE id = 'string_primary_key';

// Plain SQL
db.Find(&user, "name = ?", "jinzhu")
// SELECT * FROM users WHERE name = "jinzhu";

db.Find(&users, "name <> ? AND age > ?", "jinzhu", 20)
// SELECT * FROM users WHERE name <> "jinzhu" AND age > 20;

// Struct
db.Find(&users, UserAge: 20)
// SELECT * FROM users WHERE age = 20;

// Map
db.Find(&users, map[string]interface"age": 20)
// SELECT * FROM users WHERE age = 20;

Not 条件

db.Not("name = ?", "jinzhu").First(&user)
// SELECT * FROM users WHERE NOT name = "jinzhu" ORDER BY id LIMIT 1;

// Not In
db.Not(map[string]interface"name": []string"jinzhu", "jinzhu 2").Find(&users)
// SELECT * FROM users WHERE name NOT IN ("jinzhu", "jinzhu 2");

// Struct
db.Not(UserName: "jinzhu", Age: 18).First(&user)
// SELECT * FROM users WHERE name <> "jinzhu" AND age <> 18 ORDER BY id LIMIT 1;

// Not In slice of primary keys
db.Not([]int641,2,3).First(&user)
// SELECT * FROM users WHERE id NOT IN (1,2,3) ORDER BY id LIMIT 1;

Or 条件

db.Where("role = ?", "admin").Or("role = ?", "super_admin").Find(&users)
// SELECT * FROM users WHERE role = 'admin' OR role = 'super_admin';

// Struct
db.Where("name = 'jinzhu'").Or(UserName: "jinzhu 2", Age: 18).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' OR (name = 'jinzhu 2' AND age = 18);

// Map
db.Where("name = 'jinzhu'").Or(map[string]interface"name": "jinzhu 2", "age": 18).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' OR (name = 'jinzhu 2' AND age = 18);

选择特定字段

我们可以选择select某几个字段，从数据库返回，例如:

db.Select("name", "age").Find(&users)
// SELECT name, age FROM users;

db.Select([]string"name", "age").Find(&users)
// SELECT name, age FROM users;

db.Table("users").Select("COALESCE(age,?)", 42).Rows()
// SELECT COALESCE(age,'42') FROM users;

COALESCE函数介绍

Order

db.Order("age desc, name").Find(&users)
// SELECT * FROM users ORDER BY age desc, name;

// Multiple orders
db.Order("age desc").Order("name").Find(&users)
// SELECT * FROM users ORDER BY age desc, name;

db.Clauses(clause.OrderBy
  Expression: clause.ExprSQL: "FIELD(id,?)", Vars: []interface[]int1, 2, 3, WithoutParentheses: true,
).Find(&User)
// SELECT * FROM users ORDER BY FIELD(id,1,2,3)

Limit & Offset

db.Limit(3).Find(&users)
// SELECT * FROM users LIMIT 3;

// Cancel limit condition with -1
db.Limit(10).Find(&users1).Limit(-1).Find(&users2)
// SELECT * FROM users LIMIT 10; (users1)
// SELECT * FROM users; (users2)

db.Offset(3).Find(&users)
// SELECT * FROM users OFFSET 3;

db.Limit(10).Offset(5).Find(&users)
// SELECT * FROM users OFFSET 5 LIMIT 10;

// Cancel offset condition with -1
db.Offset(10).Find(&users1).Offset(-1).Find(&users2)
// SELECT * FROM users OFFSET 10; (users1)
// SELECT * FROM users; (users2)

Group By & Having

type result struct 
  Date  time.Time
  Total int


db.Model(&User).Select("name, sum(age) as total").Where("name LIKE ?", "group%").Group("name").First(&result)
// SELECT name, sum(age) as total FROM `users` WHERE name LIKE "group%" GROUP BY `name` LIMIT 1


db.Model(&User).Select("name, sum(age) as total").Group("name").Having("name = ?", "group").Find(&result)
// SELECT name, sum(age) as total FROM `users` GROUP BY `name` HAVING name = "group"

rows, err := db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Rows()
defer rows.Close()
for rows.Next() 
  ...


rows, err := db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Having("sum(amount) > ?", 100).Rows()
defer rows.Close()
for rows.Next() 
  ...


type Result struct 
  Date  time.Time
  Total int64

db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Having("sum(amount) > ?", 100).Scan(&results)

Distinct

//Distinct函数等同于拼接:  SELECT DISTINCT `name`,`age`
db.Distinct("name", "age").Order("name, age desc").Find(&results)

Joins

type result struct 
  Name  string
  Email string


db.Model(&User).Select("users.name, emails.email").Joins("left join emails on emails.user_id = users.id").Scan(&result)
// SELECT users.name, emails.email FROM `users` left join emails on emails.user_id = users.id

rows, err := db.Table("users").Select("users.name, emails.email").Joins("left join emails on emails.user_id = users.id").Rows()
for rows.Next() 
  ...


db.Table("users").Select("users.name, emails.email").Joins("left join emails on emails.user_id = users.id").Scan(&results)

// multiple joins with parameter
db.Joins("JOIN emails ON emails.user_id = users.id AND emails.email = ?", "jinzhu@example.org").Joins("JOIN credit_cards ON credit_cards.user_id = users.id").Where("credit_cards.number = ?", "411111111111").Find(&user)

Joins 预加载

db.Joins("Company").Find(&users)
// SELECT `users`.`id`,`users`.`name`,`users`.`age`,&
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