select the records with the highest amount

SELECT Top 1 Customer, Sum(Amount) As Total FROM
(SELECT Customer, Amount, Date FROM Sales
UNION ALL
SELECT Customer, Amount, Date FROM Services)
WHERE Date Between Date() AND Date()-30
GROUP BY Customer
ORDER BY Sum(Amount) DESC

select top 1 * from
(select SalesId ID, Rate, Quantity, 'Sales' TransactionType
 from sales
 union all
 select PurchaseId ID, Rate, Quantity, 'Purchase' TransactionType
 from purchase)
order by Rate * Quantity desc

With x as (
    Select
        b.bid_item_id,
        count(*) over (partition by b.bid_item_id) as number_of_bids,
        row_number() over (
            partition by b.bid_item_id 
            order by b.bid_amount desc
        ) as high_row,
        row_number() over (
            partition by b.bid_item_id 
            order by b.bid_amount
        ) as low_row,
        b.bid_amount,
        u.user_firstname + ' ' + u.user_lastname username
    From
        vb_bids b
            inner join
        vb_users u
            on b.bid_user_id = u.user_id
    Where
        b.bid_status = 'ok'
)
Select
    i.item_name,
    i.item_reserve,
    min(x.number_of_bids) number_of_bids,
    min(case when x.low_row = 1 then x.bid_amount end) lowest_bid,
    min(case when x.low_row = 1 then x.username end) low_bidder,
    min(case when x.high_row = 1 then x.bid_amount end) highest_bid,
    min(case when x.high_row = 1 then x.username end) high_bidder 
From
    vb_items i
        inner join
    x
        on i.item_id = x.bid_item_id
Where
    i.item_sold = 'no'
Group By
    i.item_name,
    i.item_reserve
Order By
    i.item_reserve

SELECT A.*, COUNT(B.id)
FROM TableA A
LEFT JOIN  TableB B
  ON A.id = B.id

SELECT s1.id, s1.name, s1.salary
FROM salaries s1
WHERE NOT EXISTS
(
  SELECT *
  FROM salaries s2
  WHERE s1.name = s2.name AND s1.salary < s2.salary
)

SELECT s1.id, s1.name, s1.salary
FROM salaries s1
WHERE s1.salary >=
ALL(
  SELECT salary
  FROM salaries s2
  WHERE s1.name = s2.name
)