Functions for Working with Nullable Values

isNull

Returns whether the argument is NULL.

See also operator IS NULL.

Syntax

Alias: ISNULL.

Arguments

• x — A value of non-compound data type.

Returned value

• 1 if x is NULL.
• 0 if x is not NULL.

Example

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Query:

Result:

isNullable

Returns 1 if a column is Nullable (i.e allows NULL values), 0 otherwise.

Syntax

Arguments

• x — column.

Returned value

• 1 if x allows NULL values. UInt8.
• 0 if x does not allow NULL values. UInt8.

Example

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Result:

isNotNull

Returns whether the argument is not NULL.

See also operator IS NOT NULL.

Arguments:

• x — A value of non-compound data type.

Returned value

• 1 if x is not NULL.
• 0 if x is NULL.

Example

Table:

Query:

Result:

isNotDistinctFrom

Performs null-safe comparison. Used to compare JOIN keys which contain NULL values in the JOIN ON section. This function will consider two NULL values as identical and will return true, which is distinct from the usual equals behavior where comparing two NULL values would return NULL.

Note

This function is an internal function used by the implementation of JOIN ON. Please do not use it manually in queries.

Syntax

Arguments

• x — first JOIN key.
• y — second JOIN key.

Returned value

• true when x and y are both NULL.
• false otherwise.

Example

For a complete example see: NULL values in JOIN keys.

isZeroOrNull

Returns whether the argument is 0 (zero) or NULL.

Arguments:

• x — A value of non-compound data type.

Returned value

• 1 if x is 0 (zero) or NULL.
• 0 else.

Example

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coalesce

Returns the leftmost non-NULL argument.

Arguments:

• Any number of parameters of non-compound type. All parameters must be of mutually compatible data types.

Returned values

• The first non-NULL argument
• NULL, if all arguments are NULL.

Example

Consider a list of contacts that may specify multiple ways to contact a customer.

The mail and phone fields are of type String, but the telegram field is UInt32, so it needs to be converted to String.

Get the first available contact method for the customer from the contact list:

ifNull

Returns an alternative value if the argument is NULL.

Arguments:

• x — The value to check for NULL.
• alt — The value that the function returns if x is NULL.

Returned values

• x if x is not NULL.
• alt if x is NULL.

Example

Query:

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Query:

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nullIf

Returns NULL if both arguments are equal.

Arguments:

x, y — Values to compare. Must be of compatible types.

Returned values

• NULL if the arguments are equal.
• x if the arguments are not equal.

Example

Query:

Result:

Query:

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assumeNotNull

Returns the corresponding non-Nullable value for a value of Nullable type. If the original value is NULL, an arbitrary result can be returned. See also functions ifNull and coalesce.

Arguments:

• x — The original value.

Returned values

• The input value as non-Nullable type, if it is not NULL.
• An arbitrary value, if the input value is NULL.

Example

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Query:

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toNullable

Converts the argument type to Nullable.

Arguments:

• x — A value of non-compound type.

Returned value

• The input value but of Nullable type.

Example

Query:

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Query:

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