SUBQ CONDITION FOR KEY 
SUBQ CONDITION FOR KEY is a search strategy of the SQL optimizer.
When Can the System Use This Strategy?
The database system uses this search strategy if the following conditions are met:
The table has only one primary key column.
The values for the key column come from a subquery.
or
The table has a primary key in several columns.
Equality conditions have been defined for all primary key columns that are located before the column with the subquery in the primary key.
On one column of the primary key, an IN condition is defined in the form of a subquery.
How Does the System Access the Data?
The database system performs a sequential search on part of the primary key for each results row of the subquery.