查询

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。

对单个对象使用Find而不带limit，db.Find(&user)将会查询整个表并且只返回第一个对象，这是性能不高并且不确定的。

和方法将按主键排序查找第一条和最后一条记录。仅当指向目标结构的指针作为参数传递给方法或使用指定模型时，它们才有效。此外，如果没有为相关模型定义主键，则模型将按第一个字段排序。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

根据主键检索

如果主键是数字，则可以使用内联条件使用主键检索对象。使用字符串时，需要格外小心以避免 SQL 注入;有关详细信息，请查看“安全性”部分。

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（例如，像 uuid），则查询将按如下方式编写：

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

当目标对象有一个主键值时，将使用主键构建查询条件，例如：

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;

注意： 如果您使用 gorm 的特定字段类型，例如，它将运行不同的查询来检索对象。gorm.DeletedAt

type User struct {
  ID           string `gorm:"primarykey;size:16"`
  Name         string `gorm:"size:24"`
  DeletedAt    gorm.DeletedAt `gorm:"index"`
}

var user = User{ID: 15}
db.First(&user)
//  SELECT * FROM `users` WHERE `users`.`id` = '15' AND `users`.`deleted_at` IS NULL ORDER BY `users`.`id` LIMIT 1

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';

如果已设置对象的主键，则条件查询不会涵盖主键的值，而是将其用作“and”条件。例如：

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

此查询将给出错误。因此，在要使用变量（例如从数据库中获取新值）之前，将主键属性设置为 nil。record not found``id``user

Struct & Map 条件

// 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);

注意使用 struct 查询时，GORM 将仅使用非零字段进行查询，这意味着如果您的字段的值为、或其他零值，则不会用于构建查询条件，例如：0``''``false

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

要在查询条件中包含零值，您可以使用映射，该映射将所有键值作为查询条件包含在内，例如：

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

有关更多详细信息，请参阅指定结构搜索字段。

指定结构体查询字段

使用 struct 进行搜索时，可以通过将相关字段名称或 dbname 传递给来指定要在查询条件中使用的结构中的哪些特定值，例如：Where()

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

查询条件可以内联到方法中，类似于。First``Find``Where

// 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

构建非条件，工作类似于Where

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

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);

对于更复杂的 SQL 查询。另请参阅高级查询中的组条件。

选择特定字段

Select允许您指定要从数据库中检索的字段。否则，GORM 将默认选择所有字段。

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;

另请查看智能选择字段

排序

指定从数据库中检索记录时的顺序

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;

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

限制和偏移

Limit指定要检索的最大记录数指定在开始返回记录之前要跳过的记录数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)

有关如何制作分页器的详细信息，请参阅分页

分组依据和拥有

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)

不同

从模型中选择不同的值

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

Distinct 也适用于采摘和计数

加入

指定联接条件

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

您可以将预先加载关联与单个 SQL 一起使用，例如：Joins

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`;

有条件加入

db.Joins("Company", db.Where(&Company{Alive: true})).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` AND `Company`.`alive` = true;

有关更多详细信息，请参阅预加载（预先加载）。

Joins 一个衍生表

还可以用于联接派生表。Joins

type User struct {
  Id  int
  Age int
}

type Order struct {
  UserId     int
  FinishedAt \*time.Time
}

query := db.Table("order").Select("MAX(order.finished\_at) as latest").Joins("left join user user on order.user\_id = user.id").Where("user.age > ?", 18).Group("order.user\_id")
db.Model(\&Order{}).Joins("join (?) q on order.finished\_at = q.latest", query).Scan(\&results)
// SELECT `order`.`user_id`,`order`.`finished_at` FROM `order` join (SELECT MAX(order.finished\_at) as latest FROM `order` left join user user on order.user\_id = user.id WHERE user.age > 18 GROUP BY `order`.`user_id`) q on order.finished\_at = q.latest

扫描

将结果扫描到结构中的工作方式类似于我们使用的方式Find

type Result struct {
Name string
Age  int
}

var result Result
db.Table("users").Select("name", "age").Where("name = ?", "Antonio").Scan(\&result)

// Raw SQL
db.Raw("SELECT name, age FROM users WHERE name = ?", "Antonio").Scan(\&result)

CURD接口--查询

查询