MySQL中EXISTS怎么用？一文看懂存在性查询的实战技巧

SELECT 列名  
FROM 表1  
WHERE EXISTS (子查询);

SELECT customer_id, customer_name  
FROM customers c  
WHERE EXISTS (  
    SELECT 1  
    FROM orders o  
    WHERE o.customer_id = c.customer_id  
);

-- 使用IN（可能低效）
SELECT * FROM products  
WHERE category_id IN (  
    SELECT category_id FROM categories WHERE status = 'active'  
);

-- 使用EXISTS（更高效）
SELECT * FROM products p  
WHERE EXISTS (  
    SELECT 1  
    FROM categories c  
    WHERE c.category_id = p.category_id AND c.status = 'active'  
);

SELECT customer_id, customer_name  
FROM customers c  
WHERE EXISTS (  
    SELECT 1  
    FROM orders o  
    WHERE o.customer_id = c.customer_id  
)  
AND NOT EXISTS (  
    SELECT 1  
    FROM orders o  
    WHERE o.customer_id = c.customer_id AND o.status = 'paid'  
);

SELECT student_id, student_name  
FROM students s  
WHERE EXISTS (  
    SELECT 1  
    FROM scores sc  
    JOIN courses co ON sc.course_id = co.course_id  
    WHERE sc.student_id = s.student_id  
      AND co.course_name = '数据库'  
      AND sc.score > 90  
);

-- 查找有订单的客户（使用JOIN）
SELECT DISTINCT c.customer_id, c.customer_name  
FROM customers c  
JOIN orders o ON c.customer_id = o.customer_id;

-- IN无法处理NULL值
SELECT * FROM table1  
WHERE column1 IN (SELECT column2 FROM table2);