query and query_table Functions

The query and query_table functions take a string literal, and convert it into a SELECT subquery and a table reference, respectively. Note that these functions only accept literal strings. As such, they are not as powerful (or dangerous) as a generic eval.

These functions are conceptually simple, but enable powerful and more dynamic SQL. For example, they allow passing in a table name as a prepared statement parameter:

CREATE TABLE my_table(i INT);
INSERT INTO my_table VALUES (42);

PREPARE select_from_table AS SELECT * FROM query_table($1);
EXECUTE select_from_table('my_table');

i
42

When combined with the COLUMNS expression, we can write very generic SQL-only macros. For example, below is a custom version of SUMMARIZE that computes the min and max of every column in a table:

CREATE OR REPLACE MACRO my_summarize(table_name) AS TABLE
SELECT
    unnest([*COLUMNS('alias_.*')]) AS column_name,
    unnest([*COLUMNS('min_.*')]) AS min_value,
    unnest([*COLUMNS('max_.*')]) AS max_value
FROM (
    SELECT
        any_value(alias(COLUMNS(*))) AS "alias_\0",
        min(COLUMNS(*))::VARCHAR AS "min_\0",
        max(COLUMNS(*))::VARCHAR AS "max_\0"
    FROM query_table(table_name::VARCHAR)
);

SELECT *
FROM my_summarize('https://blobs.duckdb.org/data/ontime.parquet')
LIMIT 3;

column_name	min_value	max_value
year	2017	2017
quarter	1	3
month	1	9