Dynamic projection

Prefixing a SELECT with ? turns each projected column into its own condition, keyed by the column name or alias. It affects only that SELECT's column list.

It puts a marker on each column for you, so these two templates behave identically.

?SELECT AlbumId AS Id, Title FROM albums
SELECT /*Id*/AlbumId AS Id, /*Title*/Title FROM albums

-- Title on, either form
SELECT Title FROM albums

Activate the keys like any condition key, with builder.Use("Title") or a parameter object with [ForBoolCond] members.

Always-kept columns

A ! right after the column expression keeps it out of the conditional logic. It is always projected.

?SELECT AlbumId AS Id!, Title, ArtistId FROM albums

-- nothing on
SELECT AlbumId AS Id FROM albums

-- Title on
SELECT AlbumId AS Id, Title FROM albums

! is only valid inside a ?SELECT. Anywhere else the template throws at construction.

Joining columns

&, welds columns under one key, the last column's name.

?SELECT ArtistId AS Id&, Name FROM artists

-- Name on
SELECT ArtistId AS Id, Name FROM artists

Id is not a key of its own here, it rides with Name.

Adding a key to a column

A marker before a column adds its keys on top of the column's own, combined with "and". Here Email needs both its own key and Admin.

?SELECT Id, Username, /*Admin*/Email FROM users

-- Id, Username, Email on; Admin off
SELECT Id, Username FROM users

A marker on an always-kept column replaces "always" with the marker's keys, so Id is projected when Manual is active instead of always.

?SELECT /*Manual*/Id!, Username FROM users

-- Username on, Manual off
SELECT Username FROM users

-- Username and Manual on
SELECT Id, Username FROM users

Under an outer condition

A ?SELECT can itself sit inside a bigger conditional footprint. The outer key gates the whole select, the column keys refine it.

/*Wrapping*/?SELECT Id!, Username FROM users

With Wrapping off the whole select is gone, always-kept columns included.

Across UNION

Matching column names across ?SELECTs share one key, keeping the projections in sync.

?SELECT ArtistId AS Id, Name FROM artists UNION ALL ?SELECT GenreId AS Id, Name FROM genres

-- Name on
SELECT Name FROM artists UNION ALL SELECT Name FROM genres

Different names get different keys, so mismatched aliases can produce an invalid side. Align the aliases.

In a CTE

WITH a AS (?SELECT AlbumId AS Id, Title, ArtistId FROM albums) SELECT * FROM a

-- Title on
WITH a AS (SELECT Title FROM albums) SELECT * FROM a

Modifiers and ???

A modifier before the first column is swept into that column's footprint. Isolate it with the ??? boundary.

?SELECT DISTINCT Title, Composer FROM tracks

-- only Composer on
SELECT Composer FROM tracks

?SELECT DISTINCT ??? Title, Composer FROM tracks

-- only Composer on
SELECT DISTINCT Composer FROM tracks

Mapping note

A command whose projection changes produces several result schemas. That is expected and supported. The row mapper is chosen per schema. Ask for a type whose members are all optional to fit, or a DynaObject when the shape is open-ended.