ClickHouse/docs/en/sql-reference/statements/select/with.md

# WITH Clause {#with-clause}

This section provides support for Common Table Expressions ([CTE](https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL)), so the results of `WITH` clause can be used in the rest of `SELECT` query.

## Limitations

1. Recursive queries are not supported.
2. When subquery is used inside WITH section, it’s result should be scalar with exactly one row.
3. Expression’s results are not available in subqueries.

## Examples

**Example 1:** Using constant expression as “variable”

``` sql
WITH '2019-08-01 15:23:00' as ts_upper_bound
SELECT *
FROM hits
WHERE
    EventDate = toDate(ts_upper_bound) AND
    EventTime <= ts_upper_bound
```

**Example 2:** Evicting sum(bytes) expression result from SELECT clause column list

``` sql
WITH sum(bytes) as s
SELECT
    formatReadableSize(s),
    table
FROM system.parts
GROUP BY table
ORDER BY s
```

**Example 3:** Using results of scalar subquery

``` sql
/* this example would return TOP 10 of most huge tables */
WITH
    (
        SELECT sum(bytes)
        FROM system.parts
        WHERE active
    ) AS total_disk_usage
SELECT
    (sum(bytes) / total_disk_usage) * 100 AS table_disk_usage,
    table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 10
```

**Example 4:** Re-using expression in subquery

As a workaround for current limitation for expression usage in subqueries, you may duplicate it.

``` sql
WITH ['hello'] AS hello
SELECT
    hello,
    *
FROM
(
    WITH ['hello'] AS hello
    SELECT hello
)
```

``` text
┌─hello─────┬─hello─────┐
│ ['hello'] │ ['hello'] │
└───────────┴───────────┘
```