Value expressions
Value expressions are used in a variety of contexts, such as in the target list of the SELECT
command, as new column values in INSERT
or UPDATE
, or in search conditions in a number of commands.
The result of a value expression is sometimes called a scalar, to distinguish it from the result of a table expression (which is a table).
Aggregate function calls
An aggregate function call represents the application of an aggregate function across the rows selected by a query.
An aggregate function call should be in one of the following syntaxes:
aggregate_name
is one of the aggregation functions listed on Aggregate functions, and expression
is a value expression that does not contain an aggregate expression or a window function call.
The DISTINCT
keyword, which is only available in the second form, cannot be used together with an ORDER BY
or WITHIN GROUP
clause. Additionally, it’s important to note that the order_by_clause
is positioned differently in the first and fourth forms.
In batch mode, aggregate_name
can also be in the following form:
where the AGGREGATE:
prefix converts a builtin array function (e.g. array_sum
) or an user-defined function, to an aggregate function. The function being converted must accept exactly one argument of an array type. After the conversion, a function like foo ( array of T ) -> U
becomes an aggregate function like AGGREGATE:foo ( T ) -> U
.
Window function calls
A window function call represents the application of an aggregate-like function over a set of rows that are related to the current row (the “window”).
The “window” is defined by the OVER
clause, which generally consists of three parts:
- Window partitioning (the
PARTITION BY
clause): Specifies how to partition rows into smaller sets. - Window ordering (the
ORDER BY
clause): Specifies how the rows are ordered. This part is required for ranking functions. - Window frame (the
ROWS
,RANGE
orSESSION
clause): Specifies a particular row or the range of the rows over which calculations are performed.
A window function call should be in the following syntax:
NOTE
Currently, the PARTITION BY
clause is required. If you do not want to partition the rows into smaller sets, you can work around by specifying PARTITION BY 1::int
.
For ranking window functions like row_number
, rank
and dense_rank
, ORDER BY
clause is required.
When operating in the Emit on window close mode for a streaming query, ORDER BY
clause is required for all window functions. Please ensure that you specify exactly one column in the ORDER BY
clause. This column, generally a timestamp column, must have a watermark defined for it. It’s important to note that when using the timestamp column from this streaming query in another streaming query, the watermark information associated with the column is not retained.
window_function_name
is one of the window functions listed on Window functions.
frame_clause
can be one of:
For ROWS
or RANGE
frame, frame_start
and frame_end
can be:
If only frame_start
is specified, CURRENT ROW
will be used as the end of the window.
The requirements of offset
vary in different frames. In ROWS
frame, the offset
should be a positive constant integer indicating the number of rows before or after the current row. While RANGE
frame requires the ORDER BY
clause to specify exactly one column, and the offset
expression to be a positive constant of a data type that is determined by the data type of the ordering column. For example, if the ordering column is timestamptz
, the offset
expression should be an positive constant interval
.
For SESSION
frame, the requirements of gap
are very similar to those of offset
for RANGE
frame. The ORDER BY
clause should specify exactly one column and the gap
expression should be a positive constant of a data type that is determined by the data type of the ordering column.
NOTE
Currently, SESSION
frame is only supported in batch mode and Emit-On-Window-Close streaming mode.
frame_exclusion
can be either of these:
NOTE
In RisingWave, frame_clause
is optional. Depending on whether the ORDER BY
clause is present, the default value is different. When the ORDER BY
clause is present, the default value is ROWS UNBOUNDED PRECEDING AND CURRENT ROW
. When the ORDER BY
clause is not present, the default value is ROWS UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
. This is different from the behavior in PostgreSQL. We may align the default frame with PostgreSQL in the future.
Type casts
A type cast specifies a conversion from one data type to another.
Parameter | Description |
---|---|
expression | The expression of which the data type to be converted. |
type | The data type of the returned value.For the types you can cast the value to, see [Casting](/sql/data-types/data-type-casting.md |
Row constructors
A row constructor is an expression that builds a row value using values from its member fields.
Example
The following two statements create a table and add values to the table.
The row constructor in the statement below returns all rows in table t
in the form of row values (,)
.
Array constructors
An array constructor is an expression that creates an array from a group of values.
An array can be construcuted with the following syntax.
For example:
An array constructor can be nested in another array constructor. For example:
For a nested array constructor, ARRAY
cannot be omitted. The following statement cannot be parsed.
When you create a table, define an array with square brackets. For example:
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