subquery returns more than 1 row

Question

select 
    disease_name 
from 
    disease 
where 
    disease_id=
    (select disease_id from disease_symptom where
        disease.disease_id=disease_symptom.dise         


        

Answer 1

Breaking your query down, you have

Main query:

select disease_name from disease where disease_id=

Subquery 1:

select disease_id from disease_symptom where
        disease.disease_id=disease_symptom.disease_id AND 
        symptom_id=

Sub query 2:

select symptom_id from symptom where symptom.symptom_id=disease_symptom.symptom_id
            AND symptom_name='fever' OR symptom_name='head ache'

Since you are using equal signs, the subqueries cannot return multiple items. It looks like sub query 2 has a greater chance to return 2 items due to the OR being used. You may wish to try IN clause such as WHERE symptom_id IN (sub-query2) with WHERE disease_id IN (sub-query1)