问题
this question might have been asked before, but I could not find. because I was not able to ask correctly all situations. so pardon me if it is a repeated question. I have 2 tables: tb1 - categories tb2 - tasks I want to select all tasks and quantity of tasks by each category.
What I did is this:
SELECT category_id, category_title,
(SELECT count(task_id) FROM tasks_of_app WHERE category_id ) AS counted_tasks
FROM categories_of_app
but I get all tasks' quantity for each category.
I can't use WHERE because there is not input param.
please, guys help out or tell what book to read to be able to make such kind of queries.
回答1:
WHERE category_id needs to specify what category_id should be. You need to match it with the value from the other table to make it a correlated subquery.
SELECT category_id, category_title,
(SELECT count(task_id)
FROM tasks_of_app
WHERE tasks_of_app.category_id = categories_of_app.category_id) AS counted_tasks
FROM categories_of_app
But a better way to write this is as a LEFT JOIN between the tables.
SELECT c.category_id, c.category_title, COUNT(t.task_id) AS counted_tasks
FROM categories_of_app AS c
LEFT JOIN tasks_of_app AS t ON c.category_id = t.category_id
GROUP BY c.category_id
回答2:
If there is a column category_id in tasks_of_app that references the column category_id of categories_of_app then you need a join and aggregation:
SELECT c.category_id, c.category_title, count(t.task_id) counted_tasks
FROM categories_of_app c LEFT JOIN tasks_of_app t
ON t.category_id = c.category_id
GROUP BY c.category_id, c.category_title
来源：https://stackoverflow.com/questions/59506002/selecting-counttb2-field-name-based-on-tb1-id-of-the-table1-and-tb2-field