SQL Select Only Rows Where Certain Condition is met

Question

This seems like a basic question but I just can't figure it out (Oracle SQL).

I need to pull the date of a patient visit for certain visit intervals which are contained in multiple tables, however one of these tables contains multiple visits:

Baseline table: Patient Number , Visit Date (This is the baseline visit)

Procedure table: Patient Number , Visit Date (This is the procedure visit)

Follow up table: Patient Number , Visit Interval , Visit Date (where Visit Interval contains values such as '1_month visit', '2_month visit', '3_month visit', etc.)

I want to put together a table of visit dates with each patient as a row and each visit interval as a column heading. I've tried to do this using CASE statements but I return multiple rows for a single patient.

This is what I have so far:

SELECT DISTINCT
    BASE.PT AS "Patient Number",
    BASE.VIST_DATE AS "Baseline",
    PROC.VISIT_DATE AS "Procedure",
    CASE
        WHEN FOLLOWUP.VISIT_INTERVAL = '1_Month'
        THEN FOLLOWUP.VISIT_DATE
        ELSE NULL END AS "1 Month",
    CASE
WHEN FOLLOWUP.VISIT_INTERVAL = '2_Month'
    THEN FOLLOWUP.VISIT_DATE
    ELSE NULL END AS "2 Month"
FROM BASE
LEFT JOIN PROC
    ON BASE.PT=PROC.PT
LEFT JOIN FOLLOWUP
    ON BASE.PT=FOLLOWUP.PT

This returns 2 rows per patient, with 1 row containing the Baseline, Procedure, and 1 Month visit dates (2 month column null) and the other row containing the Baseline, Procedure, and 2 Month visit dates (1 month column null). I want to display 1 row per patient with all dates populated. I'm obviously missing something (or going about it poorly in general) so I've posted for help.

Thank you!

Answer 1

You have two intervals for each patient, so you'll have two rows for each patient!

Your first row is the one with '1_month' interval, in this row you have only that column populated, and the same for the other row with '2_month' interval and measure.

For having the correct result you can use aggregates:

SELECT
    BASE.PT AS "Patient Number",
    MAX(BASE.VIST_DATE) AS "Baseline",
    MAX(PROC.VISIT_DATE) AS "Procedure",
    CASE
        WHEN FOLLOWUP.VISIT_INTERVAL = '1_Month'
        THEN FOLLOWUP.VISIT_DATE
        ELSE NULL END AS "1 Month",
    CASE
WHEN FOLLOWUP.VISIT_INTERVAL = '2_Month'
    THEN FOLLOWUP.VISIT_DATE
    ELSE NULL END AS "2 Month"
FROM BASE
LEFT JOIN PROC
    ON BASE.PT=PROC.PT
LEFT JOIN FOLLOWUP
    ON BASE.PT=FOLLOWUP.PT
GROUP BY BASE.PT