SQL queries to determine all values that would satisfy an arbitrary query - mysql

Question

I'm trying to figure out how to efficiently run a set of queries that will provide a new table of all values that would return results for an arbitrary query.
Say my table has a schema like:
id
name
age
city
What is an efficient way to list all values that would return results for an arbitrary query (e.g., SELECT * FROM main WHERE NOT city=X AND age BETWEEN Y AND Z)?
My naive approach for this would be to use a script and recurse through all possible combinations of {city, age, age} and see which SELECTs return more than 0 results, but that seems incredibly inefficient. I've also tried building large joins on {city, age, age} as well and basically using that table as an argument list to the query, but that quickly becomes an impossibility for queries on many columns.
For simple conjunctive equality queries (e.g., SELECT * FROM main WHERE name=X AND age=Y), this is much simpler, as I can do something like:
SELECT name, age, count(*) AS count FROM main GROUP BY name, age HAVING count > 0
But I'm having difficulty coming up with a general approach for anything more complicated than that.
Any pointers in the right direction would be most helpful, thanks.
EDIT:
It appears I did a very poor job explaining this, sorry.
Imagine a user gives me a database and a template query and says, "Tell me all the values I can use in this query that will yield results from this database." For example, the user might want to know all age range queries that will return at least one row (e.g., the template query is SELECT * FROM main WHERE age BETWEEN X AND Y).
In that particular example, one could run a SELECT to find the min/max ages in the database, and just tell the user to query between those ages.
Now imagine that the query template is more complicated, such as SELECT * FROM main WHERE NOT city=W AND age BETWEEN X AND Y AND name LIKE Z. How could one determine the range of W/X/Y/Z values that can be used with this query to return results? Does it require creating a join table with every single {city, age, age, name} combination and running the SELECT on each row? How can I do this efficiently so that the operation is time-bound on large databases?
Hopefully that clarifies it.

Answer 1

You could try to write a trigger after insert into your table which inserts the values into another table if they don't exist already. This way you'd have a table with the distinct values of your table. This table would look something like
columnNameFromYourTable | distinctValue
city NY
city LA
age 1
age 2
...
Then when you want to know if a record exists in your table for query SELECT * FROM main WHERE NOT city=W AND age BETWEEN X AND Y AND name LIKE Z you'd query the distinctTable with
select 1 from dual where 1=1
and not exists (select 1 from distinctTable where columnNameFromYourTable = 'city' and distinctValue = 'W')
and exists (select 1 from distinctTable where columnNameFromYourTable = 'age' and distinctValue BETWEEN X AND Y)
and exists (select 1 from distinctTable where columnNameFromYourTable = 'name' and distinctValue LIKE '%Z%')
This would be pretty fast then. If it returns 1 there is an entry in your table, if NULL not.