Key Differences Between INNER and OUTER JOINs in SQL

When you're using SQL in university to manage databases, it's important to know the differences between INNER and OUTER JOINs. This helps you get the data you need more efficiently.

INNER JOIN

• What It Is: An INNER JOIN gives you only the rows that match between two tables. If a row in one table doesn’t have a matching row in the other table, it won’t show up in the results.

• Result Set: If you have two tables, A and B, the INNER JOIN will show only the rows where the join condition is met. For example, if table A has 10 rows and table B has 5 rows, but only 3 of them match, the result will include just those 3 rows.

• Performance: INNER JOINs are usually faster because they deal with fewer rows. Studies show that INNER JOINs can be 20 to 30% more efficient than OUTER JOINs in many situations.

OUTER JOIN

OUTER JOINs come in three types: LEFT JOIN, RIGHT JOIN, and FULL JOIN.

1. LEFT JOIN:

   • What It Is: A LEFT JOIN gives you all the rows from the left table (A) and any matching rows from the right table (B). If there’s no match, it will show NULL values for the right table.
   • Result Set: If table A has 10 rows and table B has 5 with 3 matches, the result will have all 10 rows from table A. There will be 7 rows with NULLs for the right table where there’s no match.

2. RIGHT JOIN:

   • What It Is: A RIGHT JOIN gives you all the rows from the right table (B) and any matching rows from the left table (A). If there’s no match, NULL values will appear for the left table.
   • Result Set: This is similar to a LEFT JOIN. If table A has 10 rows and table B has 5 with 3 matches, the result will show all 5 rows from table B.

3. FULL JOIN:

   • What It Is: A FULL JOIN includes all the rows from both tables—LEFT and RIGHT JOIN combined. It includes NULLs for any rows that don’t have matches in either table.
   • Result Set: In this case, the result can show as many as all the rows from both tables combined, minus any matches found.

Conclusion

By knowing the main differences between INNER and OUTER JOINs, you can choose the best way to join tables. This helps you get accurate data more quickly and makes your queries work better!