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szarnyasg merged 1 commit into from
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28 changes: 17 additions & 11 deletions docs/stable/sql/expressions/star.md
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Expand Up @@ -224,16 +224,11 @@ SELECT COLUMNS(['id', 'num']) FROM numbers;
| 2 | 20 |
| 3 | NULL |

## `*COLUMNS` Unpacked Columns
## Unpacking a `COLUMNS` Expression

The `*COLUMNS` clause is a variation of `COLUMNS`, which supports all of the previously mentioned capabilities.
The difference is in how the expression expands.
By wrapping a `COLUMN` expression in `UNPACK`, the columns expand into a parent expression, much like the [iterable unpacking behavior in Python](https://peps.python.org/pep-3132/).

`*COLUMNS` will expand in-place, much like the [iterable unpacking behavior in Python](https://peps.python.org/pep-3132/), which inspired the `*` syntax.
This implies that the expression expands into the parent expression.
An example that shows this difference between `COLUMNS` and `*COLUMNS`:

With `COLUMNS`:
Without `UNPACK`, operations on the `COLUMNS` expression are applied to each column separately:

```sql
SELECT coalesce(COLUMNS(['a', 'b', 'c'])) AS result
Expand All @@ -244,18 +239,18 @@ FROM (SELECT NULL a, 42 b, true c);
|--------|-------:|-------:|
| NULL | 42 | true |

With `*COLUMNS`, the expression expands in its parent expression `coalesce`, resulting in a single result column:
With `UNPACK`, the `COLUMNS` expression is expanded into its parent expression, `coalesce` in the example above, which results in a single column:

```sql
SELECT coalesce(*COLUMNS(['a', 'b', 'c'])) AS result
SELECT coalesce(UNPACK(COLUMNS(['a', 'b', 'c']))) AS result
FROM (SELECT NULL AS a, 42 AS b, true AS c);
```

| result |
|-------:|
| 42 |

`*COLUMNS` also works with the `(*)` argument:
The `UNPACK` keyword may be replaced by `*`, [matching Python syntax](https://peps.python.org/pep-3132/), when it is applied directly to the `COLUMNS` expression without any intermediate operations.

```sql
SELECT coalesce(*COLUMNS(*)) AS result
Expand All @@ -266,6 +261,17 @@ FROM (SELECT NULL a, 42 AS b, true AS c);
|-------:|
| 42 |

> Warning In the following example, replacing `UNPACK` by `*` results in a syntax error:
>
> ```sql
> SELECT greatest(UNPACK(COLUMNS(*) + 1)) AS result
> FROM (SELECT 1 AS a, 2 AS b, 3 AS c);
> ```
>
> | result |
> |-------:|
> | 4 |

## `STRUCT.*`

The `*` expression can also be used to retrieve all keys from a struct as separate columns.
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