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copied!<p>Try this:</p> <pre><code>SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name FROM table_a LEFT OUTER JOIN table_b ON table_b.user_id = table_a.user_id LEFT OUTER JOIN table_c ON table_c.user_id = table_a.user_id WHERE 1; </code></pre> <p>It uses the <code>LEFT OUTER JOIN</code>, which means that it joins it to <code>table_b</code> on the given condition. However, for each row in <code>table_a</code>, whether it finds a matching row in <code>table_b</code> or not, it will return the <code>table_a</code> row. If it does not find a matching row, the <code>table_b</code> columns are just NULL. Same thing with <code>table_c</code>.</p> <p>Then, we just select all the <code>table_a</code> columns. However, we now have two <code>full_name</code> columns, one from <code>table_b</code> and one from <code>table_c</code>. We use <code>COALESCE</code> to merge them.</p> <p><code>COALESCE</code> returns the first non-<code>NULL</code> value.</p> <p>Since we know that there is either a matching row in <code>table_b</code> or a matching row in <code>table_c</code>, it is not a problem. However, it would be a problem if somehow you allows a matching row to be found in both <code>table_b</code> and <code>table_c</code>.</p> <p>The risk can be mitigated by adding additional <code>ON</code> clause conditions to get:</p> <pre><code>SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name FROM table_a LEFT OUTER JOIN table_b ON table_b.user_id = table_a.user_id AND table_a.user_type = 1 LEFT OUTER JOIN table_c ON table_c.user_id = table_a.user_id AND table_a.user_type = 2 WHERE 1; </code></pre> <p>Still, you will need to make sure only 1 row is present for each user in <code>table_b</code> and <code>table_c</code>.</p> <hr> <p>Instead of <code>COALESCE</code> you can optionally use <code>CASE</code> like:</p> <pre><code>SELECT table_a.*, CASE user_type WHEN 1 THEN table_b.full_name ELSE table_c.full_name END AS full_name ... </code></pre> <p>or use an <code>IF</code> function like: SELECT table_a.*, IF(user_type=1,table_b.full_name,table_c.full_name) AS full_name ...</p>

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