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/en/sql-reference/aggregate-functions/reference/singlevalueornull | 184 |
singleValueOrNull
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.
If there is only one unique value, it returns it. If there are zero or at least two distinct values, it returns NULL.
Syntax
singleValueOrNull(x)
Parameters
x
— Column of any data type.
Returned values
- The unique value, if there is only one unique non-NULL value in
x
. NULL
, if there are zero or at least two distinct values.
Examples
Query:
CREATE TABLE test (x UInt8 NULL) ENGINE=Log;
INSERT INTO test (x) VALUES (NULL), (NULL), (5), (NULL), (NULL);
SELECT singleValueOrNull(x) FROM test;
Result:
┌─singleValueOrNull(x)─┐
│ 5 │
└──────────────────────┘
Query:
INSERT INTO test (x) VALUES (10);
SELECT singleValueOrNull(x) FROM test;
Result:
┌─singleValueOrNull(x)─┐
│ ᴺᵁᴸᴸ │
└──────────────────────┘