Explicit vs. Implicit SQL Joins: What's the Difference and Which Should I Use?

SQL Joins: Explicit vs. Implicit

SQL joins merge data from multiple tables based on shared columns. Two primary join types exist: explicit and implicit.

Explicit Joins: The Modern Approach

Explicit joins utilize the JOIN keyword to clearly define the join condition. This enhances readability and maintainability. For instance:

SELECT * 
FROM table_a
INNER JOIN table_b
ON table_a.id = table_b.id;

Implicit Joins: The Older Method

Implicit joins, also called comma joins, use the comma operator (,) to combine tables, with the join condition detailed within the WHERE clause. This style is less clear and generally discouraged in modern SQL. Example:

SELECT table_a.*, table_b.*
FROM table_a, table_b
WHERE table_a.id = table_b.id;

Performance Considerations

For inner joins, explicit and implicit methods offer comparable performance in SQL Server. Factors like table size, indexing, and the specific SQL Server version significantly influence query speed.

Outdated Implicit OUTER JOINs

It's crucial to note that implicit OUTER JOIN syntax (using *= or =* in the WHERE clause after a comma) is obsolete in SQL Server 2005 and later versions. However, implicit (CROSS) JOINs using a comma (as shown above) remain supported, though explicit joins are strongly preferred for clarity.

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