如何在mysql中对生成的列使用select语句？

Question

I have 2 tables.

table_1:
Id.    Name        Surname.   company_id
1      tony.       tata.      fiat
2      Michael.    folks.     fiat
3      Salvo       ture.      alfa 

table_2:
company_id.   street.          cap.        numbers_of_guys
fiat          via le maini.    20123.      2
alfa          stazione x.      20123.      1
foo           novara           20145.      0 

I would like to create a generated column that counts how many rows I have in table_1.company_id = table_2.company_id and stores it in table_2.numbers_of_guys

How can i insert a query statment as default expression?

SELECT COUNT(table_1.id)
FROM table_2
INNER JOIN table_1 USING (company_id)

Answer 1

You shouldn't store easily calculated columns you could extend you query to include a group by.

SELECT t2.`company_id.`, t2.`street.`, t2.`cap.` , COUNT(table_1.id)
FROM table_2 t2
LEFT JOIN table_1 t1 on t1.company_id = t2.`company_id.`
group by t2.`company_id.`, t2.`street.`, t2.`cap.` 

BTW do your table column names really terminate with a full stop if so you will always need to enclose them in backticks. Also although using is syntactically correct it's better (in my view) to explicitly state both sides of the join, Also note the LEFT join.