Discover the SQL JOIN query that generates the most results. Learn why FULL OUTER JOIN maximizes data output by combining unmatched and matched rows from both tables.
Table of Contents
Question
Which of these SQL JOIN queries would yield the most possible results, assuming they run on the same database?
A.
SELECT <columns>
From TableA A
RIGHT OUTER JOIN TableB B ON A.ID = B.ID
B.
SELECT <columns>
From TableA A
LEFT OUTER JOIN TableB B ON A.ID = B.ID
C.
SELECT <columns>
From TableA A
FULL OUTER JOIN TableB B ON A.ID = B.ID
D.
SELECT <columns>
From TableA A
INNER JOIN TableB B ON A.ID = B.ID
Answer
C.
SELECT <columns>
From TableA A
FULL OUTER JOIN TableB B ON A.ID = B.ID
Explanation
SQL JOINs are used to combine data from two tables based on a related column. The type of JOIN determines how rows from the tables are matched and included in the result set. Here’s a breakdown of the options:
RIGHT OUTER JOIN (Option A)
Includes all rows from the right table (TableB) and only matching rows from the left table (TableA).
Unmatched rows in TableA are filled with NULL values.
LEFT OUTER JOIN (Option B)
Includes all rows from the left table (TableA) and only matching rows from the right table (TableB).
Unmatched rows in TableB are filled with NULL values.
FULL OUTER JOIN (Option C)
Combines the results of both LEFT OUTER JOIN and RIGHT OUTER JOIN.
Includes all rows from both tables, with NULL values for columns where no match is found.
This yields the maximum possible results, as it retains all data from both tables, regardless of matches.
INNER JOIN (Option D)
Includes only rows where there is a match between TableA and TableB.
This produces the fewest results since unmatched rows are excluded.
Why FULL OUTER JOIN Produces the Most Results
FULL OUTER JOIN retrieves:
- All matching rows between the two tables.
- All unmatched rows from both tables, filling missing values with NULL.
This ensures no data is excluded, making it ideal when you need a comprehensive view of both datasets, including gaps or missing relationships.
For example:
If TableA has 10 rows and TableB has 15 rows, with only 5 matching on ID, FULL OUTER JOIN will return:
- 5 matched rows.
- 5 unmatched rows from TableA.
- 10 unmatched rows from TableB.
Total: 20 rows.
In contrast:
- LEFT or RIGHT JOIN would return fewer results by focusing on one table’s unmatched data.
- INNER JOIN would only return the 5 matched rows.
The SQL query using FULL OUTER JOIN maximizes results by including all matched and unmatched data from both tables. This makes it the most comprehensive option among SQL JOIN types.
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