查询操作

基本查询

假设定义var user User，查询数据存入user

db.First(&user)：获取按主键升序的第一条记录
db.Take(&user)：获取一条记录，不指定排序字段
db.Last(&user)：获取按主键升序的第一条记录
db.Find(&user)：获取所有记录

First和Last只有struct传入的是指针或通过db.Model指定model才有效，另外，如果没有定义主键，则按第一个字段排序

根据主键查询

如果主键是数字类型，可以用内联条件检索

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

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

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

如果主键是字符串，查询类似如下：

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

如果目标对象有一个主键值，可以使用主键构建查询条件

var user = User{ID: 10}
db.First(&user)
// SELECT * FROM users WHERE id = 10;

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

条件

使用db.Where().xxx()在查询前添加条件，Where里用问号格式化检索条件，其余和sql语句类似，代替sql中WHERE的作用

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';

注意：若对象设置了主键，条件查询不会覆盖主键的值，而是用And相连，这样就报错了，所以记得要提前将id这样的主键设置为nil
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var user = User{ID: 10}
db.Where("id = ?", 20).First(&user)
// SELECT * FROM users WHERE id = 10 and id = 20 ORDER BY id ASC LIMIT 1

Struct & Map 条件

Where里也可以塞结构体，map和切片

// Struct
db.Where(&User{Name: "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([]int64{20, 21, 22}).Find(&users)
// SELECT * FROM users WHERE id IN (20, 21, 22);

结构体查询的时候会省略零值，如果想要保留零值需要使用map

使用结构体的时候还可以指定结构体查询字段：

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

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

内联条件

类似Where的用法，First和Find方法可以将上述条件写入方法参数

// 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, User{Age: 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

类似Where，代替sql中<>和NOT IN的作用

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(User{Name: "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([]int64{1,2,3}).First(&user)
// SELECT * FROM users WHERE id NOT IN (1,2,3) ORDER BY id LIMIT 1;

OR条件

代替sql中OR的作用

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(User{Name: "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

可以用db.Select选择从表获取的特定字段，默认获取全部字段

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;

以防你忘了：COALESCES的作用是接收多个参数，返回第一个非NULL的参数，可以更好的处理NULL值

Rows()返回的是一个*sql.Rows对象，拿到以后要手动循环读取并处理每一行数据，用法如下：
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rows, err := db.Table("users").Select("COALESCE(age, ?)", 42).Rows()
if err != nil {
    panic(err)
}
defer rows.Close()
for rows.Next() {
    var age int64
    // 用 rows.Scan 扫描当前行的值
    rows.Scan(&age)
    fmt.Println(age)
}

排序

使用db.Order()设置排序

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;

分页

使用db.Limit()和db.Offset()控制分页

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)

Groupby与Having

db.Group()代替sql里的group by操作，db.Having代替sql里的having操作

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"

Distinct

使用db.Distinct代替sql中的distinct

db.Distinct("name", "age").Order("name, age desc").Find(&results)

Join

使用db.Joins()实现JOIN操作，左外与右外在参数内部指明

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

JOINS预加载可以自动对传入的表左外连接，用一条sql查出主表和关联表的数据，同时可以选择是左外连接还是内连接

db.Joins("Company").Find(&users)
// SELECT `users`.`id`,`users`.`name`,`users`.`age`,`Company`.`id` AS `Company__id`,`Company`.`name` AS `Company__name` FROM `users` LEFT JOIN `companies` AS `Company` ON `users`.`company_id` = `Company`.`id`;

// inner join
db.InnerJoins("Company").Find(&users)
// SELECT `users`.`id`,`users`.`name`,`users`.`age`,`Company`.`id` AS `Company__id`,`Company`.`name` AS `Company__name` FROM `users` INNER JOIN `companies` AS `Company` ON `users`.`company_id` = `Company`.`id`;

得到的数据

Find类

通过Find，First方法返回的数据类型一定是一个指向模型切片或结构体的指针，GORM会根据字段名和数据库列明映射

特点是强类型，只能接收和数据库表有对应关系的模型结构体，不需要手动管理资源

Rows

来自go标准库的*sql.Rows，必须使用rows.Next()遍历，用rows.Scan()逐列扫描到基础变量，最后需要rows.Close()

rows, _ := db.Table("users").Select("name, age").Rows()
defer rows.Close()
for rows.Next() {
    var name string
    var age int
    rows.Scan(&name, &age)   // 这是 database/sql 包的 Scan
    fmt.Println(name, age)
}

特点是灵活，可以处理任意复杂动态查询，不受约束，且逐行处理适合大型结果

Scan

返回的数据类型可以是任意类型的切片或结构体指针，或者map切片

不需要模型定义，只要接受变量的结构和查询结果列名匹配

且是一次性把所有行扫描到提供的目标内

// 扫描到自定义结构体切片
type Result struct {
    Dept   string
    Count  int
}
var results []Result
db.Raw("SELECT department, COUNT(*) as count FROM users GROUP BY department").Scan(&results)

// 扫描到 map 切片
var list []map[string]interface{}
db.Table("users").Select("name").Scan(&list)

// 扫描单个列到切片
var ages []int
db.Table("users").Pluck("age", &ages)   // Pluck 是 Scan 的特殊情况

// 扫描单行单列到变量
var maxAge int
db.Table("users").Select("MAX(age)").Scan(&maxAge)

特点是比row方便且比find自由适合聚合查询和跨表查询

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【GORM】查询操作