Issues with updating a SQL table using a CASE statement - sql

Question

I'm fairly new to SQL and still trying to get my head around a few concepts...
I have created a simple table and i'm trying to update one of the columns in the table using a CASE statement. I understand why the statment won't work but i'm confused as to how to fix it....
This is my code:
CREATE TABLE q5
(
UserID INT,
Month INT,
Score INT
)
INSERT INTO q5(UserID, Month, Score)
VALUES (1,1,10), (1,2,5), (1,1,6), (2,8,6), (3,1,9), (3,4,11), (3,6,9), (4,9,10), (5,1,2);
UPDATE q5
SET
Month = CASE WHEN Month IN (1, 2, 3) THEN 'First Quarter'
WHEN Month IN (4, 5, 6) THEN 'Second Quarter'
WHEN Month IN (7, 8, 9) THEN 'Third Quarter'
ELSE 'Fourth Quarter'
END
I'm getting this error:
Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value 'First Quarter' to data type int.

Answer 1

Month is defined as an int, yet you are trying to update the table and set Month to a varchar, like 'First Quarter'. You can either change Month to store a varchar, and then modify your insert statement and WHEN Month IN pieces, or add an additional column that can store the quarter.
The query below shows the second option (adding an additional column).
CREATE TABLE q5
(
UserID INT,
Month INT,
Score INT,
Quarter varchar(20)
)
INSERT INTO q5(UserID, Month, Score)
VALUES (1,1,10), (1,2,5), (1,1,6), (2,8,6), (3,1,9), (3,4,11), (3,6,9), (4,9,10), (5,1,2);
UPDATE q5
SET
Quarter = CASE WHEN Month IN (1, 2, 3) THEN 'First Quarter'
WHEN Month IN (4, 5, 6) THEN 'Second Quarter'
WHEN Month IN (7, 8, 9) THEN 'Third Quarter'
ELSE 'Fourth Quarter'
END
Depending on how this is being utilized, you might be better off adding a View which would calculate the Quarter. This way, you wouldn't have to maintain that additional column, which is entirely dependant on the Month column.

Answer 2

Your column MONTH can only store data of type INT. What you've got now is similar to the following procedural code:
int month = 2;
if (month == 1 || month == 2 || month == 3)
{
month = "First Quarter"; // compiler error
}
As you can see, that conditionally executed line is fairly nonsensical, since month is of type int.
You're probably looking to add another column, say Quarter.
ALTER TABLE q5 ADD Quarter NVARCHAR(100)
UPDATE q5
SET
Quarter = CASE WHEN Month IN (1, 2, 3) THEN 'First Quarter'
WHEN Month IN (4, 5, 6) THEN 'Second Quarter'
WHEN Month IN (7, 8, 9) THEN 'Third Quarter'
ELSE 'Fourth Quarter'
END
Although realistically, listing the value names should be done by the UI. Best practice would be to use a computed column or separate VIEW that has a Quarter INT column. This code would add a computed column with the appropriate datatype.
ALTER TABLE q5 ADD
Quarter AS CASE WHEN Month IN (1, 2, 3) THEN 1
WHEN Month IN (4, 5, 6) THEN 2
WHEN Month IN (7, 8, 9) THEN 3
ELSE 4
END