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110 lines
3.0 KiB
Markdown
110 lines
3.0 KiB
Markdown
---
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toc_title: DISTINCT
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---
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# DISTINCT Clause {#select-distinct}
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If `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.
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You can specify the list of columns that must have unique values: `SELECT DISTINCT ON (column1, column2,...)`. If the columns are not specified, all of them are taken into consideration.
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Consider the table:
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```text
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┌─a─┬─b─┬─c─┐
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│ 1 │ 1 │ 1 │
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│ 1 │ 1 │ 1 │
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│ 2 │ 2 │ 2 │
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│ 2 │ 2 │ 2 │
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│ 1 │ 1 │ 2 │
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│ 1 │ 2 │ 2 │
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└───┴───┴───┘
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```
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Using `DISTINCT` without specifying columns:
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```sql
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SELECT DISTINCT * FROM t1;
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```
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```text
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┌─a─┬─b─┬─c─┐
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│ 1 │ 1 │ 1 │
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│ 2 │ 2 │ 2 │
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│ 1 │ 1 │ 2 │
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│ 1 │ 2 │ 2 │
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└───┴───┴───┘
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```
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Using `DISTINCT` with specified columns:
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```sql
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SELECT DISTINCT ON (a,b) * FROM t1;
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```
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```text
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┌─a─┬─b─┬─c─┐
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│ 1 │ 1 │ 1 │
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│ 2 │ 2 │ 2 │
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│ 1 │ 2 │ 2 │
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└───┴───┴───┘
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```
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## DISTINCT and ORDER BY {#distinct-orderby}
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ClickHouse supports using the `DISTINCT` and `ORDER BY` clauses for different columns in one query. The `DISTINCT` clause is executed before the `ORDER BY` clause.
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Consider the table:
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``` text
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┌─a─┬─b─┐
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│ 2 │ 1 │
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│ 1 │ 2 │
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│ 3 │ 3 │
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│ 2 │ 4 │
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└───┴───┘
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```
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Selecting data:
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```sql
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SELECT DISTINCT a FROM t1 ORDER BY b ASC;
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```
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``` text
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┌─a─┐
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│ 2 │
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│ 1 │
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│ 3 │
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└───┘
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```
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Selecting data with the different sorting direction:
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```sql
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SELECT DISTINCT a FROM t1 ORDER BY b DESC;
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```
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``` text
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┌─a─┐
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│ 3 │
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│ 1 │
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│ 2 │
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└───┘
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```
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Row `2, 4` was cut before sorting.
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Take this implementation specificity into account when programming queries.
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## Null Processing {#null-processing}
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`DISTINCT` works with [NULL](../../../sql-reference/syntax.md#null-literal) 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.
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## Alternatives {#alternatives}
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It is possible to obtain the same result by applying [GROUP BY](../../../sql-reference/statements/select/group-by.md) 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:
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- `DISTINCT` can be applied together with `GROUP BY`.
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- When [ORDER BY](../../../sql-reference/statements/select/order-by.md) is omitted and [LIMIT](../../../sql-reference/statements/select/limit.md) is defined, the query stops running immediately after the required number of different rows has been read.
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- Data blocks are output as they are processed, without waiting for the entire query to finish running.
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