Question

given the two tables and values:

Tables
People- Columns: [ID]PK Name
Field - Columns: [ID FieldName]PK FieldValue

Values
People
ID Name
1  Mary
2  John
3  Tony

Field
ID FieldName  FieldValue
1  Age        20
1  Country    USA
2  Age        21
2  Country    USA
3  Age        20
3  Country    USA

I would like a query that gives me the names of the people from USA AND have 20 years old. The result should be Mary and Tony.

SELECT name FROM people p
INNER JOIN field f ON f.ID = f.ID
WHERE
(FieldName = 'Age' AND FieldValue= '20') OR
(FieldName = 'Country' AND FieldValue= 'USA'));

The problem with that query is the “OR” between the where clauses. I will get the 3 people, instead of only Mary and Tony. If I use an AND, none result is return.

Do you have some tips?

Thanks in advance 🙂

Answer 1

What about this?

SELECT name
FROM people p
JOIN field fAge ON p.ID = fAge.ID
   AND fAge.FieldName = 'Age'
JOIN field fCountry ON p.ID = fCountry.ID
   AND fCountry.FieldName = 'Country'
WHERE fAge.FieldValue = '20'
AND fCountry.FieldValue = 'USA'

Thereby treating the Country and Age data essentially as separate tables. As it seems that the field table is used to store different types of data.

It might be useful to have different database Views on this table for each type of data. This will further simplify the above query.