Sql Server Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >=

Cracking the subquery mystery

Often, this error arises when your subquery tosses out multiple items where only one is expected. Use better filtering criteria or aggregate to handle this.

Selecting the optimal cost value

When stumbling over the error with a cost field, tailor your subquery to emit a sole cost value.

• Using Aggregate Functions:

  SELECT * FROM MainTable 
  WHERE Cost = (
    SELECT MAX(Cost) FROM SubTable 
    -- "MAX" here because we’re high-rollers--no expense spared!
    WHERE Condition)
  

• Adding Specific Criteria:

  SELECT * FROM MainTable 
  WHERE Cost = (
    SELECT Cost FROM SubTable 
    WHERE ProductID = MainTable.ProductID AND Condition 
    ORDER BY Date DESC
    -- "DESC" because the latest is always the greatest, am I right?
    )
  

Readability and aliasing

Aliasing tables and subqueries boosts readability:

SELECT mt.ID, mt.Cost 
FROM MainTable mt 
WHERE mt.Cost = (
    SELECT TOP 1 st.Cost 
    FROM SubTable st 
    WHERE st.ProductID = mt.ProductID 
    ORDER BY st.Date DESC
    -- st here stands for "sub-table", and also "star", because this code is a shining star!
)


Embracing derived tables and joins

Often, joining along with derived tables can steer clear of subqueries, particularly going through complex data.

SELECT mt.*, dt.Cost
FROM MainTable mt
JOIN (
    SELECT ProductID, MAX(Cost) AS Cost
    FROM SubTable
    GROUP BY ProductID
    -- Why use a subquery when a JOIN will do the job?
) dt ON mt.ProductID = dt.ProductID


Picking fine-tuned criteria

Examine your data, nail down why multiple values return and revise your conditions to filter data correctly.

Interplay with triggers

If triggers in your database are linked with subquery, they might lead to bumps like unexpected multi-value returns. Auditing these triggers helps, and you can rely on ALTER TABLE to disable them during your query execution.

Visualization

Imagine each subquery being a passport booth at an airport, each value representing a person:

Passport Control (Subquery): [🧔, 👩, 👴]
Plane Seat (Target Value): [🪑]

When there's only one seat left (=, !=, <, <= , >, >=):

✈️🎫🛂: [🧔] ✅ | [👩, 👴] ❌
# Only ONE passenger can board!

If multiple people try to board:

❌ Error: Too many passengers for one seat!
# This error is saying 'Subquery returned more than 1 value.'

Each subquery should act like a VIP badge—Only one person gets through, not a crowd.