The result set of an ANSI outer join depends on whether you place the restriction in the on or the where clause. The on clause defines the result set of a joined table and which rows of this joined table have null-supplied values; the where clause defines which rows of the joined table are included in the result set.
Whether you use an on or a where clause in your join condition depends on what you want your result set to include. The following examples may help you decide whether to place the predicate in the on or the where clause.
The following query places a restriction on the outer table in the where clause. Because the restriction is applied to the result of the outer join, it removes all the rows for which the condition is not true:
select title, titles.title_id, price, au_id from titles left join titleauthor on titles.title_id = titleauthor.title_id where titles.price > $20.00
title title_id price au_id ------------------- -------- ---------- ----------------- But Is It User F... PC1035 22.95 238-95-7766 Computer Phobic ... PS1372 21.59 724-80-9391 Computer Phobic ... PS1372 21.59 756-30-7391 Onions, Leeks, a... TC3218 20.95 807-91-6654 (4 rows affected)
Four rows meet the criteria and only these rows are included in the result set.
However, if you move this restriction on the outer table to the on clause, the result set includes all the rows that meet the on clause condition. Rows from the outer table that do not meet the condition are null-extended:
select title, titles.title_id, price, au_id from titles left join titleauthor on titles.title_id = titleauthor.title_id and titles.price > $20.00
title title_id price au_id -------------------- --------- ------ --------------- The Busy Executive’s BU1032 19.99 NULL Cooking with Compute BU1111 11.95 NULL You Can Combat Compu BU2075 2.99 NULL Straight Talk About BU7832 19.99 NULL Silicon Valley Gastro MC2222 19.99 NULL The Gourmet Microwave MC3021 2.99 NULL The Psychology of Com MC3026 NULL NULL But Is It User Friend PC1035 22.95 238-95-7766 Secrets of Silicon Va PC8888 20.00 NULL Net Etiquette PC9999 NULL NULL Computer Phobic and PS1372 21.59 724-80-9391 Computer Phobic and PS1372 21.59 756-30-7391 Is Anger the Enemy? PS2091 10.95 NULL Life Without Fear PS2106 7.00 NULL Prolonged Data Depri PS3333 19.99 NULL Emotional Security: PS7777 7.99 NULL Onions, Leeks, and Ga TC3218 20.95 807-91-6654 Fifty Years in Buckin TC4203 11.95 NULL Sushi, Anyone? TC7777 14.99 NULL (19 rows affected)
Moving the restriction to the on clause added 15 null-supplied rows to the result set.
Generally, if your query uses a restriction on an outer table, and you want the result set to remove only the rows for which the restriction is false, you should probably place the restriction in the where clause to limit the rows of the result set. Outer table predicates are not used for index keys if they are in the on clause.
Whether you place the restriction on an outer table in the on or where clause ultimately depends on the information you need the query to return. If you want the result set to include only the rows for which the restriction is true, place the restriction in the where clause. However, if the result set must include all the rows of the outer table, regardless of whether they satisfy the restriction, place the restriction in the on clause.
The following query includes a restriction on an inner table in the where clause:
select title, titles.title_id, titles.price, au_id from titleauthor left join titles on titles.title_id = titleauthor.title_id where titles.price > $20.00
title title_id price au_id ------------- -------- ----- ----------- But Is It U... PC1035 22.95 238-95-7766 Computer Ph... PS1372 21.59 724-80-9391 Computer Ph... PS1372 21.59 756-30-7391 Onions, Lee... TC3218 20.95 807-91-6654 (4 rows affected)
Because the restriction of the where clause is applied to the result set after the join is made, all the rows for which the restriction is not true are removed from the result set. In other words, the where clause is not true for all null-supplied values and removes them. A join that places its restriction in the where clause is effectively an inner join.
However, if you move the restriction to the on clause, it is applied during the join and is utilized in the production of the joined table. In this case, the result set includes all the rows of the inner table for which the restriction is true, plus all the rows of the outer table, which are null-extended if they do not meet the restriction criteria:
select title, titles.title_id, price, au_id from titleauthor left join titles on titles.title_id = titleauthor.title_id and price > $20.00
title title_id price au_id --------- --------- ----------- ----------- NULL NULL NULL 172-32-1176 NULL NULL NULL 213-46-8915 . . . Onions, TC3218 20.95 807-91-6654 . . . NUL NULL NULL 998-72-3567 NULL NULL NULL 998-72-3567 (25 rows affected)
This result set includes 21 rows that the previous example did not include.
Generally, if your query requires a restriction on an inner table (for example “and price > $20.00” in query above), place the condition in the on clause; this preserves the rows of the outer table. If you include a restriction for an inner table in the where clause, the result set might not include the rows of the outer table.
Like the criteria for the placement of a restriction on an outer table, whether you place the restriction for an inner table in the on or where clause ultimately depends on the result set you want. If you are interested only in the rows for which the restriction is true, place the restriction in the where clause. However, if you require the result set to include all the rows of the outer table, regardless of whether they satisfy the restriction, place the restriction in the on clause.
The restriction in the where clause of the following query includes both the inner and outer tables:
select title, titles.title_id, price, price*qty, qty from salesdetail left join titles on titles.title_id = salesdetail.title_id where price*qty > $30000.00
title title_id price qty ----------------- -------- ----- --------- ----- Silicon Valley Ga MC2222 19.99 40,619.68 2032 But Is It User Fr PC1035 22.95 45,900.00 2000 But Is It User Fr PC1035 22.95 45,900.00 2000 But Is It User Fr PC1035 22.95 49,067.10 2138 Secrets of Silico PC8888 20.00 40,000.00 2000 Prolonged Data De PS3333 19.99 53,713.13 2687 Fifty Years in Bu TC4203 11.95 32,265.00 2700 Fifty Years in Bu TC4203 11.95 41,825.00 3500 (8 rows affected)
Placing the restriction in the where clause eliminates:
The rows for which the restriction “price*qty>$30000.0” is false
The rows for which the restriction “price*qty>$30000.0” is unknown because price is null
To keep the unmatched rows of the outer table, move the restriction into the on clause:
select title, titles.title_id, price, price*qty, qty from salesdetail left join titles on titles.title_id = salesdetail.title_id and price*qty > $30000.00
title title_id price qty ----------------- -------- ----- --------- ----- NULL NULL NULL NULL 75 NULL NULL NULL NULL 75 . . . Secrets of Silico PC8888 20.00 40,000.00 2000 . . . NULL NULL NULL NULL 300 NULL NULL NULL NULL 400 (116 rows affected)
This query retains all 116 rows of the salesdetail table in the result set, and null-extends the rows that do not meet the restriction.
Where you place the restriction that includes both the inner and outer table depends on the result set you want. If you are interested in only the rows for which the restriction is true, place the restriction in the where clause. However, to include all the rows of the outer table, regardless of whether they satisfy the restriction, place the restriction in the on clause.
