SQL JOINs: USING, ON, or WHERE – Which Syntax Should You Choose?

SQL JOIN method comparison: USING, ON and WHERE

SQL provides three syntaxes to perform JOIN operations: USING, ON and WHERE. Although these syntaxes look similar, they can affect query readability, correctness, and performance.

USING syntax

The

USING syntax uses the USING keyword followed by a column name common to both tables to specify the join column:

<code class="language-sql">SELECT * FROM a JOIN b USING(ID);</code>

ON syntax

The

ON syntax uses the ON keyword and an equality expression between the joined columns to explicitly define join conditions:

<code class="language-sql">SELECT * FROM a JOIN b ON a.ID = b.ID;</code>

WHERE syntax

WHERE syntax combines join and filter conditions in a WHERE clause:

<code class="language-sql">SELECT * FROM a, b WHERE a.ID = b.ID;</code>

Performance considerations

There is no significant performance difference between these syntax options. However, due to potential ambiguity, especially with queries involving OUTER JOINs, the WHERE syntax is not recommended.

Semantic differences

The

USING syntax explicitly specifies only the join columns, which provides clarity and reduces the risk of incorrect join conditions.

The

ON syntax allows more complex join conditions, including inequality operators (=, !=, >) or additional join conditions (e.g., AND/OR).

The WHERE syntax implicitly performs an INNER JOIN based on equality of the specified columns. This can lead to unexpected Cartesian products or incorrect results for outer joins.

Example description

Consider the following query:

<code class="language-sql">FROM T1,T2,T3 WHERE T1.ID = T2.ID AND T1.foo = 'bar' AND T2.fish = 42</code>

It is not clear whether T1.foo = 'bar' is a join condition or a filter condition.

Rewriting the query using ON syntax can provide clarity:

<code class="language-sql">FROM T1
   INNER JOIN T2 ON T1.ID = T2.ID
WHERE
   T1.foo = 'bar' AND T2.fish = 42</code>

Conclusion

While the USING, ON, and WHERE syntax options can all accomplish a JOIN operation, the ON syntax is generally preferred because of its greater readability, correctness, and consistency in handling outer joins. The USING syntax provides simplicity, while the WHERE syntax should be avoided due to its potential ambiguity.

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