SQL判断数据存不存在的正确做法(99%的人还在写错!)

SELECT COUNT(*) FROM users WHERE email = 'test@example.com';

SELECT COUNT(*) FROM users WHERE email = 'zhang@example.com';

-- ✅ 推荐写法
SELECT EXISTS (
    SELECT 1 FROM users WHERE email = 'test@example.com'
) AS user_exists;

SELECT EXISTS (SELECT 1 FROM users WHERE email = 'test@example.com');
SELECT EXISTS (SELECT * FROM users WHERE email = 'test@example.com');
SELECT EXISTS (SELECT email FROM users WHERE email = 'test@example.com');

-- 检查邮箱是否已被注册
SELECT EXISTS (
    SELECT 1 FROM users 
    WHERE email = 'newuser@example.com'
) AS email_taken;
-- 返回 1 表示已被占用，0 表示可以使用

-- 找出所有有过订单的用户
SELECT u.id, u.name, u.email
FROM users u
WHERE EXISTS (
    SELECT 1 FROM orders o 
    WHERE o.user_id = u.id
);

-- 找出从来没下过单的用户
SELECT u.id, u.name, u.email
FROM users u
WHERE NOT EXISTS (
    SELECT 1 FROM orders o 
    WHERE o.user_id = u.id
);

-- 假设用户表中有 50 万条记录
-- 使用 COUNT(*) 的方式
SELECT COUNT(*) FROM users WHERE city = '上海';
-- 执行时间：150ms（需要统计所有上海的用户）
-- 使用 EXISTS 方式  
SELECT EXISTS (
    SELECT 1 FROM users WHERE city = '上海'
) AS has_sh_users;
-- 执行时间：3ms（找到第一个就直接停止了）

package main
import (
    "database/sql"
    "fmt"
    "log"
    _ "github.com/go-sql-driver/mysql"
)
// 检查邮箱是否已存在
func CheckEmailExists(db *sql.DB, email string) (bool, error) {
    var exists bool
    query := `
        SELECT EXISTS (
            SELECT 1 FROM users 
            WHERE email = ?
        )`
    err := db.QueryRow(query, email).Scan(&exists)
    if err != nil {
        return false, err
    }
    return exists, nil
}
func main() {
    // 连接数据库
    db, err := sql.Open("mysql", "user:password@tcp(localhost:3306)/mydb")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()
    // 检查邮箱是否存在
    email := "test@example.com"
    exists, err := CheckEmailExists(db, email)
    if err != nil {
        log.Fatal(err)
    }
    if exists {
        fmt.Printf("邮箱 %s 已被注册\n", email)
    } else {
        fmt.Printf("邮箱 %s 可以使用\n", email)
    }
}

// 用户注册逻辑
func RegisterUser(db *sql.DB, email, password string) error {
    // 1. 先检查邮箱是否已存在
    exists, err := CheckEmailExists(db, email)
    if err != nil {
        return fmt.Errorf("检查邮箱失败: %v", err)
    }
    if exists {
        return fmt.Errorf("邮箱 %s 已被注册", email)
    }
    // 2. 邮箱可用，执行注册逻辑
    _, err = db.Exec(`
        INSERT INTO users (email, password, created_at) 
        VALUES (?, ?, NOW())
    `, email, password)
    if err != nil {
        return fmt.Errorf("注册失败: %v", err)
    }
    fmt.Printf("用户 %s 注册成功！\n", email)
    return nil
}
// 检查用户是否有订单
func UserHasOrders(db *sql.DB, userID int) (bool, error) {
    var hasOrders bool
    query := `
        SELECT EXISTS (
            SELECT 1 FROM orders 
            WHERE user_id = ? AND status != 'cancelled'
        )`
    err := db.QueryRow(query, userID).Scan(&hasOrders)
    return hasOrders, err
}
// 获取用户信息，同时检查是否为 VIP
func GetUserWithVIPStatus(db *sql.DB, userID int) error {
    type UserInfo struct {
        ID     int    `json:"id"`
        Name   string `json:"name"`
        Email  string `json:"email"`
        IsVIP  bool   `json:"is_vip"`
    }
    var user UserInfo
    query := `
        SELECT 
            u.id,
            u.name,
            u.email,
            EXISTS (
                SELECT 1 FROM memberships m
                WHERE m.user_id = u.id 
                  AND m.status = 'active' 
                  AND m.expired_at > NOW()
            ) AS is_vip
        FROM users u
        WHERE u.id = ?`
    err := db.QueryRow(query, userID).Scan(
        &user.ID, &user.Name, &user.Email, &user.IsVIP,
    )
    if err != nil {
        return err
    }
    fmt.Printf("用户信息: %+v\n", user)
    return nil
}

-- 为了让 EXISTS 查询更快，记得在经常查询的字段上建索引
CREATE INDEX idx_users_email ON users(email);
CREATE INDEX idx_orders_user_id ON orders(user_id);

-- 如果字段可能为 NULL，记得特殊处理
SELECT EXISTS (
    SELECT 1 FROM users 
    WHERE phone IS NOT NULL 
      AND phone = '13800138000'
) AS phone_exists;

-- 没必要排序
SELECT EXISTS (
    SELECT 1 FROM users 
    WHERE city = '上海'
    ORDER BY created_at  -- 这个排序完全没啥卵用
);
-- 直接开撸
SELECT EXISTS (
    SELECT 1 FROM users 
    WHERE city = '上海'
);