MySQL join with a subquery

Question

I have three tables and am trying to get info from two and then perform a calculation on the third and display all the results in one query.

The (simplified) tables are:

table: employee_work

employee_id name
1 Joe
2 Bob
3 Jane
4 Michelle

table: carryover

employee_id days
1 5
2 10
3 3

table: timeoff

employee_id time_off_type days
1 Carryover 2
1 Leave 3
1 Carryover 1
2 Sick 4
2 Carryover 4
3 Leave 1
4 Sickness 4

The results I would like are:

employee_id, carryover.days, timeoff.days
1 5 3
2 10 4
3 3 0

However when I run the query, whilst I get the correct values in columns 1 and 2, I get the same number repeated in the third column for all entries.

Here is my query:

Select
  employee_work.employee_id,
  carryover.carryover,
  (SELECT SUM(days) FROM timeoff WHERE timeoff.time_off_type = 'Carryover' 
  AND timeoff.start_date>='2013-01-01') AS taken
From
  carryover Left Join
  employee_work On employee_work.employee_id = carryover.employee_id Left Join
  timeoff On employee_work.employee_id = timeoff.employee_id Left Join
Where
  carryover.carryover > 0
Group By
  employee_work.employee_id

I have tried to group by in the sub query but I then get told "Subquery returns more than one row" - how can I ensure that the sub query is respecting the join so it only looks at each employee at a time so I get my desired results?

Answer 1

The answer to your question is to use a correlated subquery. You don't need to mention the timeoff table twice in this case:

Select
  employee_work.employee_id,
  carryover.carryover,
  (SELECT SUM(days)
   FROM timeoff
   WHERE timeoff.time_off_type = 'Carryover' and
         timeoff.start_date>='2013-01-01' and
         timeoff.employee_id = employee_work.employee_id
  ) AS taken
From
  carryover Left Join
  employee_work On employee_work.employee_id = carryover.employee_id
Where
  carryover.carryover > 0
Group By
  employee_work.employee_id;

An alternative structure is to do the grouping for all employees in the from clause. You can also remove the employee_work table, because it does not seem to be being used. (You can use carryover.employee_id for the id.)

Select co.employee_id, co.carryover, et.taken
From carryover c Left Join
     (SELECT employee_id, SUM(days) as taken
      FROM timeoff
      WHERE timeoff.time_off_type = 'Carryover' and
            timeoff.start_date>='2013-01-01'
     ) et
     on co.employee_id = et.employee_id
 Where c.carryover > 0;

I don't think the group by is necessary. If it is, then you should probably have an aggregation function in the original query.