Nice, that is what I was looking for. Of course, it'd need to point to production data as well, so maybe test_case is null, in that case:

  CREATE TABLE data_a AS (SELECT 'a' AS test_case, 1 AS value);
  CREATE TABLE data_b AS (SELECT 'b' AS test_case, 2 AS value);
  CREATE TABLE data_prod AS (SELECT NULL AS test_case, prod_table.value FROM prod_table);

  CREATE VIEW data AS (SELECT * FROM data_a UNION ALL SELECT * FROM data_b UNION ALL SELECT * FROM data_prod);

  CREATE VIEW complicated_query AS (SELECT test_case, value+1 FROM data);

  -- when testing
  SELECT * FROM complicated_query WHERE test_case = 'a';
  SELECT * FROM complicated_query WHERE test_case = 'b';

  -- when in 'production'
  SELECT * FROM complicated_query WHERE test_case IS NULL;

You just reinvented defunctionalization, which is a transformation from a domain that has first-class values to a domain where support is only second-class. Defunctionalization is typically used in programming languages to simulate first-class functions in languages where functions are only second-class citizens, like C and Pascal.

This perfectly illustrates my point. You had to manually defunctionalize your data model and queries to support what I'm saying should be inherently part of SQL.

> languages where functions are only second-class citizens, like C and Pascal.

1) Only if you define Pascal as only Wirth's very first version. That changed almost immediately.

2) Only if you refuse to equate “pointer to function” with “function”. Which in C, where “everything is a pointer” (a bit like in Unix Linux “everything is a file”), seems rather silly.