This statement indicates that SQL Server needs to insert some of the query results into an intermediate worktable, and later in the query processing will then select values from that table. This statement is most often seen with a query that involves a GROUP BY clause; the results are first put into a work table, and then the qualifying rows in the work table are grouped based on the given column in the GROUP BY clause.
The following query returns a list of all cities and indicates the number of authors who live in each city. The query plan is composed of two steps: the first step selects the rows into a worktable, and the second step retrieves the grouped rows from the worktable:
SELECT city, total_authors = count(*) FROM Authors GROUP BY city
STEP 1 The type of query is SELECT (into a worktable) GROUP BY Vector Aggregate FROM TABLE authors Nested iteration Table Scan TO TABLE Worktable 1 STEP 2 The type of query is SELECT FROM TABLE Worktable 1 Nested iteration Table Scan