How to Resolve the 'Subquery Returned More than 1 Value' Error in SQL Server?

Troubleshooting the "Subquery Returned More Than One Value" Error in SQL Server

SQL Server subqueries used within comparison operators (like '=') must yield a single value. Multiple values trigger the error: "Subquery returned more than 1 value." Let's explore solutions.

Debugging Steps:

1. Subquery Result Validation: The subquery should return only one row and one column. Carefully examine the query for ambiguous or duplicate data that could produce multiple results.

2. Join Condition Review: Ensure the join conditions between the main query and the subquery create a one-to-one relationship. Remove redundant or unnecessary join criteria.

3. Aggregate/Grouping Functions: If your subquery uses aggregate (like SUM, AVG, COUNT) or grouping functions, confirm their correct application and that they generate a single output value.

Example and Solution:

The provided example demonstrates a subquery returning multiple cost values, causing the error. The solution uses TOP 1 to restrict the subquery's output to a single row:

<code class="language-sql">SELECT
    orderdetails.sku,
    orderdetails.mf_item_number,
    orderdetails.qty,
    orderdetails.price,
    supplier.supplierid,
    supplier.suppliername,
    supplier.dropshipfees,
    cost = (SELECT TOP 1 supplier_item.price
           FROM   supplier_item
           WHERE  supplier_item.sku = orderdetails.sku
                  AND supplier_item.supplierid = supplier.supplierid)
FROM   orderdetails
       INNER JOIN supplier ON supplier.supplierid = orderdetails.mfr_id
       INNER JOIN group_master ON group_master.sku = orderdetails.sku  
WHERE  invoiceid = '339740'</code>

Using TOP 1 guarantees only one row (and thus one price) is returned, resolving the error. Note that TOP 1 without an ORDER BY clause returns an arbitrary row; consider adding ORDER BY for consistent results if the order matters.

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