Sql where if not exist

The “NOT EXISTS” statement uses a subquery to filter out records that do not exist in the underlying subquery. The following example finds employees who are not in departments which have names that start with P. BusinessEntityID = p. Person AS p JOIN HumanResources. It is used to restrict the number of rows returned by the SELECT Statement. Let’s consider we want to select all students that have no grade lower than 9.

For this, we can use NOT EXISTS, which negates the logic of the EXISTS operator. Therefore, the NOT EXISTS operator returns true if the underlying subquery returns no record. What does exist mean in SQL? Performance with a Free Demo. So within the same transaction as the insert we can determine if the cust_id already exists.

This has to be wrapped in a transaction to avoid a race condition, though. As you can see, EXISTS allows us to easily check on multiple columns, which is not possible with IN. By prefixing the operators with the NOT operator, we negate the Boolean output of those operators.

The EXISTS operator is a logical operator that allows you to check whether a subquery returns any row.

Using NOT IN for example will return all rows with a value that cannot be found in a list. EXISTS returns true if the subquery returns one or more records. WHERE EXISTS tests for the existence of any records in a subquery. The last example is a double-nested NOT EXISTS query.

EXISTS is commonly used with correlated subqueries. That is, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it the question “ does a city exist with a store that is not in Stores ”? But it is easier to say that a nested NOT EXISTS the question “ is x TRUE for all y? It can be used in a SELECT, INSERT, UPDATE, or DELETE statement. SQL : EXISTS Condition. This is not the case however with 1-many or many-many.

A JOIN would cause a worktable to be created in the backgroun whereas exists would not. This means that the NOT EXISTS statement will return false every single time if there are any records in the table at all, regardless of the state of any rows from the outer query. It will not compare each row in the outer query to each row in the subquery. If a subquery returns any rows at all, EXISTS subquery is TRUE, and NOT EXISTS subquery is FALSE.

You can list only one table in the FROM clause in the DELETE statement. So, if you need to perform a DELETE operation based on the values in another table, easiest way is to use the EXISTS or NOT EXISTS. They produce the safe efficient plans with some kind of an Anti Join.

However, single line insert as I showed is probably better in high concurrency.