Question

我试图找到哪些日子没有主菜（recipe_course = 18），但我得到的只是零结果。我错过了什么？

Two tables
table recipes{recipe_id, recipe_name, recipe_course}
table meals{meal_id, meal_date, meal_recipe}

Select `meal_date`
From `meals` mls
Join `recipes` rrr on mls.`meal_recipe` = rrr.`recipe_id`
Where rrr.`recipe_course` = 18
Group by `meal_date`
Having Count(`recipe_course`) = 0

Answer 1

最简单的方法是使用＆＃34;而不是＆＃34;。＆＃34; =查找没有recipe_course 18＆＃34;

的所有用餐日期

select m.meal_date
from meals m
where m.meal_recipe not in (
  select r.recipe_id
  from recipes r
  where r.recipe_course = 18
)

Answer 2

或者，使用LEFT JOIN ... IS NULL：

SELECT  m.meal_date
    FROM  meals m
    LEFT JOIN  recipes r  ON r.recipe_id = m.meal_recipe
      AND  r.recipe_course = 18
    WHERE  r.recipe_id IS NULL

也可以使用NOT EXISTS ( SELECT * ... )

MariaDB Find Where Count（）= 0

2 个答案: