How to compute one column based off multiple tables in mysql - mysql

Question

sql import file
--sources (id, name)---
1, "source one"
2, "source two"
3, "source three"
--first_related_items (value, source_id) ---
0, 1
--second_related_items (value, source_id) ---
0, 2
2, 3
--third_related_items (value, source_id) ---
1, 3
I have source rows and related items in multiple other tables. How do I get a computed column based off the other tables? The computed column would be for each row in source table. Like
SELECT
sources.name as name,
COMPUTED_VALUE(
first_related_items,
second_related_items,
third_related_items) as result
FROM sources
The computed column would be derived something like this
if(value for source row in first table == 0){
return 0
}
else if(value for source row in second table == 0){
if(value for source row in third table == 1){
return 1
} else {
return 0
}
} else if(value for source row in third table != NULL){
return value for source row in third table
} else {
return -1
}

Answer 1

You can join all four tables and compute this column as a case expression:
SELECT s.id,
CASE WHEN fri.value = 0 THEN 0
WHEN sri.value = 0 THEN CASE WHEN tri = 1 THEN 1 ELSE 0 END
WHEN tri IS NOT NULL THEN tri
ELSE -1
END AS computed_value
FROM sources s
LEFT JOIN first_related_items fri ON fri.source_id = s.id
LEFT JOIN second_related_items sri ON sri.source_id = s.id
LEFT JOIN third_related_items tri ON tri.source_id = s.id