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

Try this:

SELECT
    od.Sku,
    od.mf_item_number,
    od.Qty,
    od.Price,
    s.SupplierId,
    s.SupplierName,
    s.DropShipFees,
    si.Price as cost
FROM
    OrderDetails od
    INNER JOIN Supplier s on s.SupplierId = od.Mfr_ID
    INNER JOIN Group_Master gm on gm.Sku = od.Sku
    INNER JOIN Supplier_Item si on si.SKU = od.Sku and si.SupplierId = s.SupplierID
WHERE
    od.invoiceid = '339740'

This will return multiple rows that are identical except for the cost column. Look at the different cost values that are returned and figure out what is causing the different values. Then ask somebody which cost value they want, and add the criteria to the query that will select that cost.

Check to see if there are any triggers on the table you are trying to execute queries against. They can sometimes throw this error as they are trying to run the update/select/insert trigger that is on the table.

You can modify your query to disable then enable the trigger if the trigger DOES NOT need to be executed for whatever query you are trying to run.

ALTER TABLE your_table DISABLE TRIGGER [the_trigger_name]

UPDATE    your_table
SET     Gender = 'Female'
WHERE     (Gender = 'Male')

ALTER TABLE your_table ENABLE TRIGGER [the_trigger_name]

SELECT COLUMN 
    FROM TABLE 
WHERE columns_name
    IN ( SELECT COLUMN FROM TABLE WHERE columns_name = 'value');

note: when we are using sub-query we must focus on these points:

1. if our sub query returns 1 value in this case we need to use (=,!=,<>,<,>....)
2. else (more than one value), in this case we need to use (in, any, all, some )