Investigate use cases for nested query builders

[jakeboone02]

Example using v6.3.0: https://codesandbox.io/s/snowy-snow-ub85bl?file=/src/CustomRule.tsx

Based on this Discord chat.

Use cases

• SQL EXISTS clauses

Misc. thoughts

• Store the query from the nested query builder in the value property of the parent rule.
• Nested query builders should inherit all props from their parent query builder by default?
• Store additional/override props for the nested query builder in the nested property of fields or operators.
• Trigger the display of a nested query builder based on truthiness of nested property, or maybe a new value source selection ("query"?).

Questions

• What should formatQuery do with rules that have nested queries? Can they be integrated somehow like a sub-group, or would they always need a custom ruleProcessor?
  • Use case for EXISTS clauses would need some more thought. Initially:

    WHERE EXISTS (SELECT 1 FROM [???] WHERE [nested query rules here])
    --                   ^?       ^?         ^ the easy part, maybe