使用案例表达式的SQL查询

Question

Let's say I have the following table

+----------+--------+-------+-------+
| Employee | Day    | Task  | Hours |
+----------+--------+-------+-------+
| Chip     | 01 Jan | TaskA | 2     |
+----------+--------+-------+-------+
| Chip     | 01 Jan | TaskB | 3     |
+----------+--------+-------+-------+
| Chip     | 02 Jan | TaskB | 4     |
+----------+--------+-------+-------+

And I want to generate the following table using an SQL query

+----------+--------+-------------+
| Employee | Day    | TaskA_Hours |
+----------+--------+-------------+
| Chip     | 01 Jan | 2           |
+----------+--------+-------------+
| Chip     | 02 Jan | null        |
+----------+--------+-------------+

I am trying to achieve this by the following code but that's not working. I understand why and can think of a solution using a left join but I am tempted to think that there is a smarter solution out there

SELECT 
Employee,
Day,
CASE when Task = 'TaskA' THEN Hours ELSE null END AS TaskA_Hours

FROM new_table

Any ideas?

Answer 1

Just add aggregation

SELECT 
  Employee,
  Day,
  SUM(CASE when Task = 'TaskA' THEN Hours END) AS TaskA_Hours
FROM new_table
GROUP BY Employee, Day

Answer 2

SQL Fiddle

MS SQL Server 2017 Schema Setup :

CREATE TABLE new_table
    ([Employee] varchar(4), [Day] varchar(10), [Task] varchar(5), [Hours] int)
;

INSERT INTO new_table
    ([Employee], [Day], [Task], [Hours])
VALUES
    ('Chip', '01 Jan', 'TaskA', 2),
    ('Chip', '01 Jan', 'TaskB', 3),
    ('Chip', '02 Jan', 'TaskB', 4)
;

Query 1 :

SELECT 
  Employee,
  Day,
  SUM(CASE when Task = 'TaskA' THEN Hours END) AS TaskA_Hours,
  SUM(CASE when Task = 'TaskB' THEN Hours END) AS TaskB_Hours
FROM new_table
GROUP BY Employee, Day

Results :

| Employee |    Day | TaskA_Hours | TaskB_Hours |
|----------|--------|-------------|-------------|
|     Chip | 01 Jan |           2 |           3 |
|     Chip | 02 Jan |      (null) |           4 |