How to Handle Subqueries Returning More Than One Row?

Addressing the "subquery returns more than 1 row" Error

When dealing with subqueries, it is crucial to understand their potential to return multiple rows. This can lead to errors if not handled appropriately, as in the case of the "subquery returns more than 1 row" error.

Subqueries Returning Multiple Values

The crux of this issue lies in the fact that the subquery in question returns multiple rows. When conditions are specified in the main query that depend on values obtained from the subquery, this error arises.

Using IN for Multiple Values

To resolve this issue, the IN operator should be employed. Unlike the equality (=) operator, which expects a single value, IN allows for multiple values to be specified. The syntax for using IN with a subquery is:

select * <br>from table<br>where id IN (subquery);<br>

In this example, the subquery will return multiple values, and the id column of the table will be matched against any of those values.

Example:

For instance, consider the following queries:

SELECT *<br>FROM Students<br>WHERE Marks = (SELECT MAX(Marks) FROM Students);  -- Returns only 1 value</p>
<p>SELECT *<br>FROM Students<br>WHERE Marks IN</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">  (SELECT Marks 
   FROM Students 
   ORDER BY Marks DESC
   LIMIT 10);                              -- Returns 10 values

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In the first query, the subquery returns the maximum mark, which is a single value. Therefore, the equality operator (=) can be used. In the second query, however, the subquery returns the top 10 marks, which are multiple values. Hence, the IN operator is required.

By employing IN, you can effectively handle subqueries that return multiple rows, ensuring that the criteria in the main query are met appropriately, resolving the "subquery returns more than 1 row" error effectively.

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