the TableExpression in the select command constructs an intermediate virtual table by possibly combining tables, views, eliminating rows, grouping, etc. This table is finally passed on to processing by the select list. The Selection determines which columns of the intermediate table are actually output.

Like select but takes an NP list of Expressions instead of a general Selection.

After the select list has been processed, the result table can be subject to the elimination of duplicate rows using selectDistinct.

Like selectDistinct but takes an NP list of Expressions instead of a general Selection.

selectDistinctOn keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT ON expressions are interpreted using the same rules as for ORDER BY. ORDER BY is used to ensure that the desired row appears first.

The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s). The ORDER BY clause will normally contain additional expression(s) that determine the desired precedence of rows within each DISTINCT ON group.

In order to guarantee they match and reduce redundancy, this function will prepend the The DISTINCT ON expressions to the ORDER BY clause.

Like selectDistinctOn but takes an NP list of Expressions instead of a general Selection.

The simplest kinds of Selection are Star and DotStar which emits all columns that a TableExpression produces. A select List is a list of Expressions. A Selection could be a list of WindowFunctions Over WindowDefinition. Additional Selections can be selected with Also.