• Simple analysis of exists and in in SQL Server


    The in and exists functions are similar. However, due to different optimization schemes, not exists is usually faster than not in, because not exists can use the combination algorithm. Second, not in cannot, while exists is not as fast as in, because in may use more combination algorithms at this time. As shown in the […]

  • Explain SQL exists operator in detail


    Exists operator The exists operator is used to determine whether there are records in the query clause. If one or more records exist, it returns true; otherwise, it returns false. SQL exists syntax SELECT column_name(s) FROM table_name WHERE EXISTS (SELECT column_name FROM table_name WHERE condition); Demo database In this tutorial, we will use the runoob […]

  • Detailed explanation of the impact of not exists on external layer query in Oracle


    Preface Recently, my colleagues found a problem: the buffer get in 12C is very high, but the buffer get in 10g is very low. Doubt there is a problem with the optimizer of 12C. In this 10g environment and 12C environment, the amount of data is roughly the same, but there are only a few […]

  • The easiest way to implement SQL with multiple identical contents and only one


    The following situations are often encountered in SQL. In a table, there are two records that are basically the same, and one or several fields are slightly different, At this time, we may need to kick out these different data, that is, only one of two or more records is kept. As follows: table timeand […]

  • Use of Exists in SQL


    For example, in the Northwind database, a query is SELECT c.CustomerId,CompanyName FROM Customers c WHERE EXISTS( SELECT OrderID FROM Orders o WHERE o.CustomerID=c.CustomerID) How does EXISTS work here? The sub-query returns the OrderID field, but the outside query is looking for the Customer ID and CompanyName fields. These two fields are definitely not in the […]