Cannot display name instead of id in two fields using left join

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table1: (patient_id, patient_name, complaint_id_right, complaint_id_left)
table2: (complaint_id, complaint_name)

I have two tables as above, one for patients(table1) and other as list table (table2)

I cannot display complaint_name instead of both complaint_id_right and complaint_id_left at the sametime, because i use table2 as list for both fields (complaint_id_right, complaint_id_left).
How to do it?

What I have tried:

select patient_id, patient_name, complaint_name, complaint_name
from table1
left join table2
on table1.complaint_id_right = table2.complaint_id and table1.complaint_id_left = table2.complaint_id

Posted 11-Jan-21 18:32pm

hanymet

Updated 11-Jan-21 22:22pm

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Maciej Los · Accepted Answer · 2021-01-11T22:22:00

Another way is to use Common Table Expressions[^].

SQL

;WITH CTE AS
(
  SELECT pa.patient_id, pa.patient_name, 'left' comp_type, co.complaint_name
  FROM table1 pa LEFT JOIN table2 co ON pa.complaint_id_left = com.complaint_id
  UNION ALL
  SELECT pa.patient_id, pa.patient_name, 'right' comp_type, co.complaint_name
  FROM CTE pa LEFT JOIN table2 co ON pa.complaint_id_right = table2.complaint_id 
)
SELECT *
FROM CTE

For further details, please see:
WITH common_table_expression (Transact-SQL) - SQL Server | Microsoft Docs[^]
Mastering Common Table Expression or CTE in SQL Server[^]

Good luck!

OriginalGriff · Accepted Answer · 2021-01-11T20:30:00

You need to do a separate JOIN per "link".
Try this:

SQL

SELECT p.patient_id, p.patient_name, cl.complaint_name, cr.complaint_name
  FROM table1 p
LEFT JOIN table2 cr
  ON p.complaint_id_right = cr.complaint_id
LEFT JOIN table2 cl 
  ON p.complaint_id_left = cl.complaint_id

But do yourself a favour and use more sensible names! "table1" should be "Patients", "table2" should be "Complaints", "patient_id" should be "ID", "complaint_id" should be "ID" and so on. You don't need to include the table name in its ID column since you will always refer to it via the table name anyway.

I also suspect that your design is flawed - "left" and "right" columns should probably be a third table linking patients to complaints:
HasComplaint:

ID           (IDENTITY or UNIQUEIDENTIFIER)
PatientID    (Foreign key to Patients.ID)
ComplaintID  (Foreign key to Complaints.ID)

This allows for a patient with one complaint, two complaints, or a dozen complaints.

Cannot display name instead of id in two fields using left join

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