OR

IN

AND

NOT

SELECT COUNT(*) FROM users WHERE WEEKDAY(register_date) >= 5

SELECT COUNT(*) FROM users WHERE DAYOFWEEK(register_date) IN (1, 7)

SELECT COUNT(*) FROM users WHERE DAYNAME(register_date) IN ('Saturday', 'Sunday')

SELECT COUNT(*) FROM users WHERE DAYOFWEEK(register_date) IN (6, 7)

CASE WHEN允许在一个查询中根据多个条件返回不同的值。

CASE WHEN表达式必须以END关键字结束。

如果没有匹配的WHEN条件，且提供了ELSE子句，则ELSE子句的值会被返回。

CASE WHEN只能用于SELECT语句中，不能用于WHERE、ORDER BY或HAVING子句。

SELECT s.name FROM students s 
WHERE s.id IN (SELECT sc.student_id FROM scores sc WHERE sc.score > 80 AND sc.course_id IN (SELECT id FROM courses WHERE name = '数学'));

SELECT s.name FROM students s, scores sc, courses c 
WHERE s.id = sc.student_id AND sc.course_id = c.id AND c.name = '数学' AND sc.score > 80;

SELECT s.name FROM students s 
JOIN scores sc ON s.id = sc.student_id 
JOIN courses c ON sc.course_id = c.id 
WHERE c.name = '数学' AND sc.score > 80;

SELECT s.name FROM students s 
WHERE EXISTS (SELECT 1 FROM scores sc JOIN courses c ON sc.course_id = c.id WHERE sc.student_id = s.id AND c.name = '数学' AND sc.score > 80);

RANK() OVER (PARTITION BY employee_id, MONTH(sale_date) ORDER BY sale_amount DESC)

DENSE_RANK() OVER (PARTITION BY employee_id, MONTH(sale_date) ORDER BY sale_amount DESC)

DENSE_RANK() OVER (PARTITION BY MONTH(sale_date) ORDER BY sale_amount DESC)

ROW_NUMBER() OVER (PARTITION BY employee_id, MONTH(sale_date) ORDER BY sale_amount DESC)

HAVING COUNT(user_id) >= 2

以上都可以，但A最简单直接

HAVING COUNT(order_id) >= 2

HAVING COUNT(DISTINCT order_id) >= 2