FROM-less SELECT

CamusDB supports SELECT statements without a FROM clause. The statement evaluates the projection list against one synthetic row, so it returns exactly one row unless LIMIT / OFFSET skips it.

This is useful for application probes, ORM-generated utility SQL, parameter checks, scalar function calls, casts, and simple existence checks that do not need an outer table.

Supported Forms

Evaluate scalar expressions:

SELECT 1 + 1;
SELECT upper("abc");
SELECT CAST("5" AS INT64);
SELECT CASE WHEN 1 = 1 THEN "ok" ELSE "fail" END;
SELECT regexp_split_to_array("one,two,three", ",");

Bind parameters:

SELECT @value;

Return multiple projections:

SELECT 41 + 1 AS answer, "ok" AS status;

When a projection is not aliased, CamusDB uses ordinal column names such as 0, 1, and 2.

Use LIMIT and OFFSET to keep or skip the single synthetic row:

SELECT 1 LIMIT 0;
SELECT 1 LIMIT 1 OFFSET 1;

Both examples return zero rows.

Projection Subqueries

A subquery inside the projection of a FROM-less SELECT is evaluated first and then replaced with its scalar result:

SELECT EXISTS (
  SELECT 1
  FROM accounts
  WHERE email = @email
);

NOT EXISTS is also supported:

SELECT NOT EXISTS (
  SELECT 1
  FROM accounts
  WHERE email = "nobody@example.com"
);

Scalar subqueries can be compared or returned directly:

SELECT (SELECT COUNT(*) FROM accounts) > 0;

SELECT (SELECT COUNT(*) FROM accounts);

The subquery can also read from a derived table:

SELECT (
  SELECT COUNT(*)
  FROM (
    SELECT 1
    FROM accounts
    WHERE email = @email
  ) AS matched
) > 0;

Because a FROM-less SELECT has no outer row, these projection subqueries are uncorrelated. Correlated projection subqueries in table-backed SELECT statements are a separate query shape.

Rejected Shapes

FROM-less SELECT is intentionally limited to projection evaluation plus optional LIMIT and OFFSET.

Query shape	Result
SELECT *	Rejected because * requires a table source.
SELECT COUNT(*)	Rejected because aggregates require a table source. Use a scalar subquery such as SELECT (SELECT COUNT(*) FROM accounts).
SELECT 1 WHERE ...	Rejected. Add a FROM clause when you need WHERE.
SELECT 1 GROUP BY ...	Rejected. Aggregation requires a table source.
SELECT 1 HAVING ...	Rejected. Aggregation requires a table source.
SELECT 1 ORDER BY ...	Rejected. There is only one synthetic row.
SELECT missing_name	Fails with UnknownColumn because the identifier cannot be resolved.

EXPLAIN

Plain EXPLAIN for a FROM-less SELECT returns a fixed plan shape:

EXPLAIN SELECT 41 + 1 AS answer;

Typical output shape:

physical  project          answer
physical  constant-source  1 row

When LIMIT or OFFSET is present, a limit node is included.

EXPLAIN (ANALYZE) is rejected for FROM-less SELECT because there is no table access to measure. Use plain EXPLAIN for this query shape.