Only one expression can be specified in the select list when the subquery is not introduced with EXISTS

Counts, Toppings and Other Solo Acts: Subquery Solutions

When Doubles Aren't Double the Fun

There are times when a subquery yields twins, or duplicate entries, that disrupt the one-column disposition. Here, DISTINCT clears the duplicate doppelgängers, returning a unique value from the subquery:

WHERE id = (SELECT DISTINCT id FROM SubqueryTable WHERE condition); -- Witnessing the birth of a unicorn 🦄
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All for One: Aggregate Data and Groups

In cases where aggregate data is critical for evaluation, employ COUNT and GROUP BY within your subquery. Make sure to aggregate data into a singular value that the parent query can easily digest:

SELECT CustomerID, COUNT(*)
FROM Orders
GROUP BY CustomerID
HAVING COUNT(*) = (SELECT TOP 1 COUNT(*) FROM Orders GROUP BY CustomerID ORDER BY COUNT(*) DESC); -- All for one and one for all!
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Top of the Class: Filtering by Percentile

To address scenarios that involve selection based on a specific percentile, TOP (0.1) PERCENT is a great companion in your subquery. Despite its role, it maintains a single result category:

WHERE ProductPrice > (SELECT TOP (0.1) PERCENT AVG(ProductPrice) FROM Products ORDER BY ProductPrice DESC); -- The 1% we don't mind!
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JSON Fix for Multi-column Dilemma

In the event where the subquery is designed to return JSON data, ensure that FOR JSON PATH is used appropriately. This allows your subquery to format data as JSON without violating the one-column rule:

SELECT JSON_Query((SELECT name, price FROM Products FOR JSON PATH)) AS ProductInfo FROM MainTable; -- SQL meets JSON in harmony 🎶
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Filtering & Sorting: Two Steps, One Leap

Inner Order May Not Reflect Outer Peace

While sorting within subqueries may often be redundant, there could be situations where it's non-negotiable. In such cases, use an ORDER BY clause within your subquery. It's independent of the main query's order:

SELECT name
FROM Employees
WHERE EXISTS (
    SELECT 1
    FROM Sales
    WHERE Sales.EmployeeID = Employees.ID
    ORDER BY Amount DESC
); -- The inner peace...err...order!
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EXISTS: A Lifesaver for Multi-column Subqueries

When your subquery necessitates returning multiple columns, introduce EXISTS into your command. This keyword checks for row existence without directly comparing values:

SELECT *
FROM MainTable m
WHERE EXISTS (
    SELECT *
    FROM SubqueryTable s
    WHERE m.id = s.id
    AND m.name = s.name
); -- Who needs multiple columns? We've got EXISTS!
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Independent Testing of Subqueries: Because We Like Being Sure

Before you launch your subquery into the wider world of your main query, test it independently. This ensures it delivers the results you expect:

SELECT id FROM SubqueryTable WHERE condition; -- Probing the subquery. Feeling like a scientist already!
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The Harmony of Subquery and Main Query

Ensure that the conditions in your subquery are in lockstep with your main query's WHERE clause - this ensures the subquery truly complements the main query:

SELECT Name
FROM Products
WHERE Category = (SELECT DISTINCT Category FROM Categories WHERE SpecialFlag = 1); -- Symbiotic SQL!
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Streamlining for Performance

Identify and iron out redundant operations within your subquery to enhance overall query performance. A leaner subquery can contribute to a mightier, more efficient main query:

SELECT *
FROM MainTable
WHERE id IN (
    SELECT id
    FROM SubqueryTable
    WHERE condition -- Beware: Potential Redundancy Alert!
);
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With these insights, the role of subqueries as data filters becomes clearer, helping you stride confidently towards cleaner, error-free SQL code.