# 查询
## 检索单个对象
GORM 提供 `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。当没有找到记录时，它会返回错误 `ErrRecordNotFound`
```go
// 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
// 检查 ErrRecordNotFound 错误
errors.Is(result.Error, gorm.ErrRecordNotFound)
```
## 检索对象
```go
// 获取全部记录
result := db.Find(&users)
// SELECT * FROM users;
result.RowsAffected // 返回找到的记录数，相当于 `len(users)`
result.Error // returns error
```
## 条件
### String 条件
```go
// 获取第一条匹配的记录
db.Where("name = ?", "jinzhu").First(&user)
// SELECT * FROM users WHERE name = 'jinzhu' ORDER BY id LIMIT 1;
// 获取全部匹配的记录
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
```go
// 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;
// 主键切片条件
db.Where([]int64{20, 21, 22}).Find(&users)
// SELECT * FROM users WHERE id IN (20, 21, 22);
```
**注意** 当使用结构作为条件查询时，GORM 只会查询非零值字段。这意味着如果您的字段值为 `0`、`''`、`false` 或其他 [零值](https://tour.golang.org/basics/12)，该字段不会被用于构建查询条件，例如：
```go
db.Where(&User{Name: "jinzhu", Age: 0}).Find(&users)
// SELECT * FROM users WHERE name = "jinzhu";
```
您可以使用 map 来构建查询条件，例如：
```go
db.Where(map[string]interface{}{"Name": "jinzhu", "Age": 0}).Find(&users)
// SELECT * FROM users WHERE name = "jinzhu" AND age = 0;
```
### 内联条件
用法与 `Where` 类似
```go
// 根据主键获取记录（仅适用于整型主键）
db.First(&user, 23)
// SELECT * FROM users WHERE id = 23;
// 根据主键获取记录，如果是非整型主键
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 NOT 条件，用法与 `Where` 类似
```go
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;
// 不在主键切片中的记录
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
```go
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';
```
您还可以在高级查询中查看 [Group 条件](./advanced_query.html)，更轻松地编写复杂 SQL
## 选择特定字段
选择您想从数据库中检索的字段，默认情况下会选择全部字段
```go
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;
```
## Order
指定从数据库检索记录时的排序方式
```
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;
```
## Limit & Offset
`Limit` 指定获取记录的最大数量 `Offset` 指定在开始返回记录之前要跳过的记录数量
```go
db.Limit(3).Find(&users)
// SELECT * FROM users LIMIT 3;
// 通过 -1 消除 Limit 条件
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;
// 通过 -1 消除 Offset 条件
db.Offset(10).Find(&users1).Offset(-1).Find(&users2)
// SELECT * FROM users OFFSET 10; (users1)
// SELECT * FROM users; (users2)
```
## Group & Having
```go
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`
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()
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()
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
从模型中选择不相同的值
```go
db.Distinct("name", "age").Order("name, age desc").Find(&results)
```
`Distinct` 也可以配合 [`Pluck`](http://v2.gorm.io/zh_CN/docs/advanced_query.html#pluck), [`Count`](http://v2.gorm.io/zh_CN/docs/advanced_query.html#count) 使用
## Joins
指定 Joins 条件
```go
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)
// 带参数的多表连接
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
您可以使用 `Joins` 实现单条 SQL 预加载关联记录，例如：
```go
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`;
```
参考 [预加载](./preload.html) 了解详情
## Scan
Scan 结果至 struct，用法与 `Find` 类似
```go
type Result struct {
Name string
Age int
}
var result Result
db.Table("users").Select("name", "age").Where("name = ?", "Antonio").Scan(&result)
// 原生 SQL
db.Raw("SELECT name, age FROM users WHERE name = ?", "Antonio").Scan(&result)
```
