1.What is the purpose of using DISTINCT on multiple columns in SQL? DISTINCT on multiple columns eliminates rows where all selected fields are identical, ensuring unique combinations of the specified columns in the result set. 2.How does DISTINCT work with multiple columns? When used with multipl...
ANSI_NULLS = ON, ANSI_PADDINGS = ON, ANSI_WARNINGS = ON, ARITHABORT = ON, CONCAT_NULL_YIELDS_NULL = ON, QUOTED_IDENTIFIER = ON, NUMERIC_ROUNDABORT = OFF. 44、尽量将数据的处理工作放在服务器上,减少网络的开销,如使用存储过程。以前由于SQL SERVER对复杂的数学计算不支持,所以不得不将这个工作放在...
(13.x) RTM or Service Pack 1 has a compatibility level of 90, which isn't supported for other databases. This doesn't have an effect on the functionality of replication. Upgrading to later service packs and versions of SQL Server will result in the compatibi...
如何比较二个同样功能的语句的性能好坏呢,我建议采用二种方法: 1. 直接把二个查询语句放在【SQL Server Management Studio】,然后去看它们的【执行计划】,SqlServer会以百分比的方式告诉你二个查询的【查询开销】。这种方法简单,通常也是可以参考的,不过,有时也会不准,具体原因请接着往下看(可能索引统计信息过旧)。
SQL Server 2016 (13.x) 藉由將字串數據行的述詞向下推送至 SCAN 節點,大幅改善了使用字串型數據行的分析查詢效能。 String predicate pushdown leverages the primary/secondary dictionary created for columns to improve the query performance. 例如,請考慮數據列群組內包含100個相異字串值的字串...
DISTINCT aggregates where the distinct columns are compatible with the ORDER clause. The ORDER clause doesn't guarantee ordered results when a SELECT query is executed, unless ORDER BY is also specified in the query. See sys.function_order_columns (Transact-SQL) for information on how to query...
尽量选择区分度高德列作为索引,区分度公式count(distinct col)/count(*),表示字段不重复的比例。 索引列不能使用函数或参与计算,不能进行类型转换,否则索引会失效。 尽量扩展索引,不要新建索引。 【 减少查询字段,避免回表查询 】 回表查询就是先定位主键值,在根据主键值定位行记录,需要扫描两遍索引。解决方案:只...
SQLSERVER2012 列存储索引的简单研究和测试 SQLSERVER2012 列存储索引的简单研究和测试 转自:微软技术大会2016 逐行插入:插入数据-》行组 超过100K 102400行才会插入到压缩行组-》压缩行组-》移动元组-》列段 大批量插入:插入数据 超过100K 102400行-》压缩行组-》移动元组-》列段...
The following example applies multiple query hints to query_id 39, includingRECOMPILE,MAXDOP 1, and the SQL Server 2012 (11.x) query optimizer behavior: SQL EXEC sys.sp_query_store_set_hints @query_id = 39, @query_hints = N'OPTION(RECOMPILE, MAXDOP 1,USEHINT (''QUERY_OPTIMIZER_COMPATI...
Case Statement in Where clause with parameters SQL Server CASE statement inclusion and exclusions case statement inside a where clause with 'IN' operator CASE Statement on multiple columns CASE STATEMENT RETURNING MULTIPLE ROWS Case Statement returning multiple values CASE statement returns "Invalid Column...