T-SQL：检查表中是否存在数据

Question

I am using a view that checks if data exists in another table and if it exists if shows the number of row's connected to that table. Now I was wondering if there might be a faster way of doing this, since I am only interested if there is data and not necessary how many data rows there are. I guess when it does not need to count it will be faster.

This is what I use now:

SELECT
    dbo.user.id,
    dbo.user.userCode,
    COALESCE (TotalProducts.ProductsInback, 0) AS ProductsInback
FROM 
    dbo.user
LEFT OUTER JOIN
    (SELECT COUNT(id_product) AS ProductsInback, userCode
     FROM dbo.Product
     GROUP BY userCode) AS TotalProducts ON dbo.Product.userCode = TotalProducts.userCode
WHERE 
    dbo.user.userCode = 'XYZ' 

Now this works all fine an it gives me the number of products connected to the user XYZ that are in the back of the store. However I just want to know if the user has products in the back of the store, I don't need to know how many. That seems to me a faster solution (walking anyway to the back of the store). So replacing the COUNT with ... ?

Answer 1

You are right, for a lookup whether data exists in another table, we use EXISTS or IN , because we don't have to go through all matching rows then, but can stop at the first one found.

EXISTS :

SELECT
  id,
  userCode,
  CASE WHEN EXISTS (SELECT * FROM dbo.Product p WHERE p.userCode = u.userCode )
       THEN 1 ELSE 0 END AS ProductsInback
FROM dbo.user u
WHERE u.userCode = 'XYZ' 

IN :

SELECT
  id,
  userCode,
  CASE WHEN userCode IN (SELECT userCode FROM dbo.Product)
       THEN 1 ELSE 0 END AS ProductsInback
FROM dbo.user
WHERE userCode = 'XYZ' 

Answer 2

If you change your left join to an inner join, you will just get a list of users with products. The other users will not appear on the list.

SELECT
dbo.user.id,
dbo.user.userCode

FROM dbo.user

JOIN dbo.Product
  ON dbo.Product.userCode= TotalProducts.userCode