Use Select data in nested Select statement in MySQL

Question

I have a query in MySQL.

Full Query

SELECT 
  tc.expense AS expense,
  tc.tour_sub_code,
  tc.login_id

FROM
  tc_wallet tc 
WHERE tc.login_id = 'vinod.kumbala' 
  AND tc.expense = 'Daily Allowance' 
  AND tc.delete_flag = 'F' 
  AND tc.status != 'reject' 

Result

Expense              Tour_sub_code          login_id 

DAILY ALLOWANCE       MOS-EUROPE100119      vinod.kumbala
DAILY ALLOWANCE       Test                  vinod.kumbala

Initially I am fetching data from tc_wallet table.
Now my requirement is that I need to find the total count of attendances for a particular tour_sub_code. Attendances count can be found from attendance_master table for that particular tour_sub_code. So I included a nested select query which is

Nested select

(SELECT 
    COUNT(*) 
  FROM
    (SELECT 
      * 
    FROM
      `attendance_master` 
    WHERE `delete_flag` = 'F' 
      AND login_id = 'vinod.kumbala' 
      AND `tour_sub_code` = tc.`tour_sub_code` 
    GROUP BY `device_date`) t1) AS newNoOfdays 

Full Query

SELECT 
      tc.expense AS expense,
      tc.tour_sub_code,
      tc.login_id,
      (SELECT 
        COUNT(*) 
      FROM
        (SELECT 
          * 
        FROM
          `attendance_master` 
        WHERE `delete_flag` = 'F' 
          AND login_id = 'vinod.kumbala' 
          AND `tour_sub_code` = tc.`tour_sub_code` 
        GROUP BY `device_date`) t1) AS newNoOfdays 

    FROM
      tc_wallet tc 
    WHERE tc.login_id = 'vinod.kumbala' 
      AND tc.expense = 'Daily Allowance' 
      AND tc.delete_flag = 'F' 
      AND tc.status != 'reject' 

Now this query gives me error as

Unknown column 'tc.tour_sub_code' in 'where clause'

Expected Result

Expense              Tour_sub_code          login_id            Count

    DAILY ALLOWANCE       MOS-EUROPE100119      vinod.kumbala     20
    DAILY ALLOWANCE       Test                  vinod.kumbala     44

Can I know where I am going wrong with this ?
Also is there any other way i get result like by using JOINS ?

Answer 1

In the innermost subquery you are referencing a column from the outer most query. This is not permitted for subqueries in the FROM clause (derived tables). However - You don't need that subquery. What you need is COUNT(DISTINCT device_date)

Rewrite

(SELECT 
    COUNT(*) 
  FROM
    (SELECT 
      * 
    FROM
      `attendance_master` 
    WHERE `delete_flag` = 'F' 
      AND login_id = 'vinod.kumbala' 
      AND `tour_sub_code` = tc.`tour_sub_code` 
    GROUP BY `device_date`) t1) AS newNoOfdays 

to

(SELECT 
    COUNT(DISTINCT device_date)
  FROM `attendance_master` 
  WHERE `delete_flag` = 'F' 
    AND login_id = 'vinod.kumbala' 
    AND `tour_sub_code` = tc.`tour_sub_code` 
) AS newNoOfdays 

You can also rewrite the full query to a LEFT JOIN query:

SELECT 
  tc.expense AS expense,
  tc.tour_sub_code,
  tc.login_id,
  COUNT(DISTINCT device_date) AS newNoOfdays 
FROM tc_wallet tc
LEFT JOIN attendance_master am
  ON  am.tour_sub_code = tc.tour_sub_code
  AND am.delete_flag = 'F' 
  AND am.login_id = 'vinod.kumbala' 
WHERE tc.login_id = 'vinod.kumbala' 
  AND tc.expense = 'Daily Allowance' 
  AND tc.delete_flag = 'F' 
  AND tc.status != 'reject'