How can i use exists in sql

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Video Course. Hypersistence Optimizer. ERP Contact. By continuing to use this website, you agree to their use. To find out more, including how to control cookies, see here: Our Cookie Policy. The following example compares two queries that are semantically equivalent. The following example shows two queries to find stores whose name is the same name as a vendor. The following example shows queries to find employees of departments that start with P. The following example finds employees who are not in departments which have names that start with P.

The following example identifies whether any rows in the ProspectiveBuyer table could be matches to rows in the DimCustomer table.

The query will return rows only when both the LastName and BirthDate values in the two tables match. What I don't understand is how does the outer query know which row the subquery is checking?

For example:. I understand that if the id from the supplier and orders table match, the subquery will return true and all the columns from the matching row in the suppliers' table will be outputted. What I don't get is how the subquery communicates which specific row lets say the row with supplier id 25 should be printed if only a true or false is being returned.

For 'each' row from Suppliers , check if there 'exists' a row in the Order table that meets the condition Suppliers. But, the application is interested in displaying the full name of a student , not just the identifier, so we need info from the student table as well.

The outer query selects the student row columns we are interested in returning to the client. The database engine does not have to run the subquery entirely. This query says, in effect:. Ponies comment is correct. You'd need to do grouping with that join, or select distinct depending on the data you need. What you describe is a so called query with a correlated subquery. In general it's something that you should try to avoid by writing the query by using a join instead:. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 10 years, 6 months ago. Active 10 months ago. Viewed 98k times. I came across this quote about "exists" and don't understand something: Using the exists operator, your subquery can return zero, one, or many rows, and the condition simply checks whether the subquery returned any rows.

It appears to me that there is no relationship between the outer query and the subquery. Improve this question. Veer Shrivastav 5, 11 11 gold badges 46 46 silver badges 81 81 bronze badges. Dan Dan 1 1 gold badge 7 7 silver badges 3 3 bronze badges. Add a comment.