Map Reducing a Set of Values Into a Dynamic SQL UNION Query

Sounds fancy, right? But it’s a really nice and reasonable approach to doing dynamic SQL with jOOQ.

This blog post is inspired by a Stack Overflow question, where a user wanted to turn a set of values into a dynamic UNION query like this:

 SELECT T.COL1 FROM T WHERE T.COL2 = 'V1' UNION SELECT T.COL1 FROM T WHERE T.COL2 = 'V2' ... UNION SELECT T.COL1 FROM T WHERE T.COL2 = 'VN' 

Note, both the Stack Overflow user and I are well aware of the possibility of using IN predicates :-), let’s just assume for the sake of argument, that the UNION query indeed outperforms the IN predicate in the user’s particular MySQL version and database. If this cannot be accepted, just imagine a more complex use case.

The solution in Java is really very simple:

 import static org.jooq.impl.DSL.*; import java.util.*; import org.jooq.*; public class Unions { public static void main(String[] args) { List<String> list = Arrays.asList("V1", "V2", "V3", "V4"); System.out.println( list.stream() .map(Unions::query) .reduce(Select::union)); } // Dynamically construct a query from an input string private static Select<Record1<String>> query(String s) { return select(T.COL1).from(T).where(T.COL2.eq(s)); } } 

The output is:

 Optional[( select T.COL1 from T where T.COL2 = 'V1' ) union ( select T.COL1 from T where T.COL2 = 'V2' ) union ( select T.COL1 from T where T.COL2 = 'V3' ) union ( select T.COL1 from T where T.COL2 = 'V4' )] 

If you’re using JDK 9+ (which has Optional.stream()), you can further proceed to running the query fluently as follows:

 List<String> list = Arrays.asList("V1", "V2", "V3", "V4"); try (Stream<Record1<String>> stream = list.stream() .map(Unions::query) .reduce(Select::union)) .stream() // Optional.stream()! .flatMap(Select::fetchStream)) { ... } 

This way, if the list is empty, reduce will return an empty optional. Streaming that empty optional will result in not fetching any results from the database.

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