Postgres SQL：获取组数

Question

I have the following table

>> tbl_category

id | category
-------------
0  | A
1  | B
...|...

>>tbl_product

id | category_id | product
---------------------------
0  | 0           | P1 
1  | 1           | P2
...|...          | ...

I can use the following query to count the number of products in a category.

select category, count(tbl.product) from tbl_product 
join tbl_category on tbl_product.category_id = category.id 
group by catregory

However, there are some categories that never have any product belonging to. How do I get these to show up in the query result as well?

Answer 1

Use a left join :

select c.category, count(tbl.product) 
from tbl_category c left join
     tbl_product p
     on p.category_id = c.id 
group by c.category;

The table where you want to keep all the rows goes first ( tbl_category ).

Note the use of table aliases to make the query easier to write and to read.