Does an INNER JOIN After an OUTER JOIN Negate the OUTER JOIN's Effect?

Nested SQL Joins: INNER JOINs and OUTER JOINs

Does an INNER JOIN following an OUTER JOIN cancel out the OUTER JOIN's effect? And should INNER JOINs always precede OUTER JOINs in multi-join queries?

The answer is nuanced. A subsequent INNER JOIN will only negate the OUTER JOIN if its ON clause requires the presence of columns that could be NULL due to the OUTER JOIN. Simply rearranging the joins won't fix this; you'll need to change the INNER JOIN to a suitable OUTER JOIN.

Let's illustrate:

Example 1 (Reordering Doesn't Matter):

<code class="language-sql">SELECT *
FROM person
LEFT JOIN address
  ON person.address_id = address.id
INNER JOIN email
  ON person.email_id = email.id</code>

Here, reordering the joins won't change the results.

Example 2 (Reordering Matters):

<code class="language-sql">SELECT *
FROM person
LEFT JOIN address
  ON person.address_id = address.id
INNER JOIN city
  ON address.city_id = city.id</code>

The second ON clause necessitates a non-NULL address.city_id. This defeats the LEFT OUTER JOIN, filtering out person records without a matching city via address. To correct this, replace the INNER JOIN with a LEFT OUTER JOIN.

Best Practices:

While not strictly mandatory, it's generally recommended to place INNER JOINs before OUTER JOINs. This improves readability and clarifies the query's essential filtering conditions. Furthermore, RIGHT OUTER JOINs are often less preferred and can usually be rewritten using LEFT OUTER JOINs for better clarity and maintainability.

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