SELECT DISTINCT is specified, only unique rows will remain in a query result. Thus only a single row will remain out of all the sets of fully matching rows in the result.
DISTINCT works with NULL as if
NULL were a specific value, and
NULL==NULL. In other words, in the
DISTINCT results, different combinations with
NULL occur only once. It differs from
NULL processing in most other contexts.
It is possible to obtain the same result by applying GROUP BY across the same set of values as specified as
SELECT clause, without using any aggregate functions. But there are few differences from
GROUP BY approach:
DISTINCTcan be applied together with
- When ORDER BY is omitted and LIMIT is defined, the query stops running immediately after the required number of different rows has been read.
- Data blocks are output as they are processed, without waiting for the entire query to finish running.
ClickHouse supports using the
ORDER BY clauses for different columns in one query. The
DISTINCT clause is executed before the
ORDER BY clause.
┌─a─┬─b─┐ │ 2 │ 1 │ │ 1 │ 2 │ │ 3 │ 3 │ │ 2 │ 4 │ └───┴───┘
When selecting data with the
SELECT DISTINCT a FROM t1 ORDER BY b ASC query, we get the following result:
┌─a─┐ │ 2 │ │ 1 │ │ 3 │ └───┘
If we change the sorting direction
SELECT DISTINCT a FROM t1 ORDER BY b DESC, we get the following result:
┌─a─┐ │ 3 │ │ 1 │ │ 2 │ └───┘
2, 4 was cut before sorting.
Take this implementation specificity into account when programming queries.