Joe Celko's SQL for Smarties, 3rd Edition

17.6. The UNION JOIN

The UNION JOIN was defined in Standard SQL, but I know of no SQL product that has implemented it. As the name implies, it is a cross between a UNION and a FULL OUTER JOIN. The definition followed easily from the other infixed JOIN operators. The syntax has no searched clause:

<table expression 1> UNION JOIN <table expression 2>

The statement takes two dissimilar tables and puts them into one result table. It preserves all the rows from both tables and does not try to consolidate them. Columns that do not exist in one table are simply padded out with NULLs in the result rows. Columns with the same names in the tables have to be renamed differently in the result. It is equivalent to:

<table expression 1> FULL OUTER JOIN <table ...