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58 lines
1.5 KiB
Markdown
58 lines
1.5 KiB
Markdown
---
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slug: /en/sql-reference/aggregate-functions/reference/singlevalueornull
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sidebar_position: 184
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---
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# singleValueOrNull
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The aggregate function `singleValueOrNull` is used to implement subquery operators, such as `x = ALL (SELECT ...)`. It checks if there is only one unique non-NULL value in the data.
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If there is only one unique value, it returns it. If there are zero or at least two distinct values, it returns NULL.
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**Syntax**
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``` sql
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singleValueOrNull(x)
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```
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**Parameters**
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- `x` — Column of any [data type](../../data-types/index.md) (except [Map](../../data-types/map.md), [Array](../../data-types/array.md) or [Tuple](../../data-types/tuple) which cannot be of type [Nullable](../../data-types/nullable.md)).
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**Returned values**
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- The unique value, if there is only one unique non-NULL value in `x`.
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- `NULL`, if there are zero or at least two distinct values.
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**Examples**
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Query:
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``` sql
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CREATE TABLE test (x UInt8 NULL) ENGINE=Log;
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INSERT INTO test (x) VALUES (NULL), (NULL), (5), (NULL), (NULL);
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SELECT singleValueOrNull(x) FROM test;
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```
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Result:
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```response
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┌─singleValueOrNull(x)─┐
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│ 5 │
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└──────────────────────┘
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```
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Query:
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```sql
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INSERT INTO test (x) VALUES (10);
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SELECT singleValueOrNull(x) FROM test;
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```
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Result:
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```response
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┌─singleValueOrNull(x)─┐
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│ ᴺᵁᴸᴸ │
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└──────────────────────┘
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```
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