对于MySQL 来说,是存在关联缓存的,缓存的大小可以由join_buffer_size参数进行设置,在 MySQL 中,对于同一个 SQL 多关联(join)一个表,就会多分配一个关联缓存,如果在一个 SQL 中关联的表越多,所占用的内存也就越大。如果程序中大量的使用了多表关联的操作,同时join_buffer_size设置的也不合理的情况下,就容易造...
SQL Server Use DISTINCT in COALESCE to remove duplicate valueI suspect that theSubCategoryvalue for...
SQL Server Use DISTINCT in COALESCE to remove duplicate valueI suspect that theSubCategoryvalue for...
When I use DISTINCT in an INNER JOIN query that returns more than 1 column in the SELECT a full table scan is performed rather than using an index. When 1 column is returned in the SELECT the index is used. (1) Create the following tables: ...
Select count (distinct id) from sql_distinct; In the above example, we can see that this statement will ignore the count of null values. It will only retrieve the unique count of not null values. We can also retrieve the unique count of a number of columns by using the statement. In ...
我们先来看一个test case,这个案例根据客户真实案例改编,模拟的是在表关联条件复杂的情况下,优化器对表关联后的结果集估值过小,可能使用错误的执行计划,希望通过增加use_hash hint来优化SQL。 --创建4个表 create table tv as select rownum as id,a.* from dba_objects a; ...
“conditions” names the conditions that must be met in order to select the records If you don’t want to remove duplicate data, or if you’re sure that there will be no duplicates in the result set, then you can use ALL instead of DISTINCT. But ALL is the default keyword in SQL st...
selectdistinctf.Currencykeyfrom[dbo].[FactInternetSales]f innerjoindimcurrencyd onf.currencykey=d.currencykey Conclusion about SQL intersect In this article, we learned the SQL intersect concept. We remind the set theory to understand the SQL intersect concept and then we show examples and common...
delete distinct () ... is not supported in Sql server. Wednesday, February 20, 2013 12:28 AM I tryed this : delete from employee where Name in (select distinct(Name) from employee) but it delete all the records delete from employee where Name NOT in (select distinct(Name) from ...
We can use SQL SELECT Distinct to find unique references of data. For example, we have a result set that shows different rows of a table and each row consists of columns. Now sometimes we are more interested in unique patterns of the data rather than the data itself which is one of the...