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