Joining TablesWhen you select columns from more than one table. (Folders on the left) you are performing a join. The database has to have some way of knowing how you want to align the columns of data because they may not share the same key value. Upon adding columns from multiple tables the join panel will appear. In this panel you select a column from both tables which must match for a record to appear in your query return. Also, you select which column to consider first. This may be important if one table's column has values that do not appear in the other column. In that case the first considered column will appear, along with other columns from that table, and the columns from the other table will return NULL values. Most of the time you will select a corresponding unique key from each table in the join box. |