alter table student add index index_name (name); 因为name是索引列,这里假设有两个叫张三的,ref级别规则就是能查出多个或0个,很显然能查出来多个,那这条SQL语句,必然是ref级别! 执行结果: 数据: 五.range级别 检索指定范围的行,查找一个范围内的数据,where后面是一个范围查询 (between,in,> < >=); 注:
语句为 where name = ‘kaka’ and name is null,name 为普通索引。 index_merge:查询语句使用了俩个以上的索引,常见在使用 and、or 会出现,官方文档将此类型放在 ref_or_null 之后,但是在很多的情况下由于读取索引过多性能有可能还不如 range。 unique_subquery:用于 where 中的 in 查询,完全替换子查询,效...
例如有如下语句执行时: value in(select primary_key from single_table where ...) 对于in子句来说,当in子句里的子查询返回的是某一个表的主键时,type显示为unique subquery。 8、index_subquery当有如下语句执行时: value in(select key_column from single_table where ...) 与上面的相似,表示对于in子句来...
range:范围扫描; index:索引树扫描; ALL:全表扫描(full table scan); 画外音:这些是最常见的,大家去explain自己工作中的SQL语句,95%都是上面这些类型。 上面各类扫描方式由快到慢: system > const > eq_ref > ref > range > index > ALL 下面一一举例说明。 一、system explain select * from mysql.tim...
Microsoft.SqlServer.TransactSql.ScriptDom 検索 Microsoft.SqlServer.TransactSql.ScriptDom Microsoft.SqlServer.TransactSql.ScriptDom AbortAfterWaitType AcceleratedDatabaseRecoveryDatabaseOption AddAlterFullTextIndexAction AddFileSpec AddMemberAlterRoleAction AddSearchPropertyListAction AddSensitivityClassificationStateme...
この章では、次のSQL文について説明します。CREATE TYPE CREATE TYPE BODY CREATE USER CREATE VIEW DELETE DISASSOCIATE STATISTICS DROP CLUSTER DROP CONTEXT DROP DATABASE DROP DATABASE LINK DROP DIMENSION DROP DIRECTORY DROP DISKGROUP DROP FLASHBACK ARCHIVE DROP FUNCTION DROP INDEX DROP INDEXTYPE DROP ...
Sets the data item link type between data items in a query to determine the records that are included in the resulting data set.
In SQL Server, the instance of and cast as XQuery expressions on SequenceTypes are partially supported. instance of Operator The instance of operator can be used to determine the dynamic, or run-time, type of the value of the specified expression. For example: Copy Expression instance of ...
AzureSqlDWLinkedServiceTypeProperties withCommandTimeout(Object commandTimeout) Set the commandTimeout property: The default wait time (in seconds) before terminating the attempt to execute a command and generating an error, used by recommended ver...
Applies to:SQL Server Oracle data types and Microsoft SQL Server data types do not always match exactly. Where possible, the matching data type is selected automatically when publishing an Oracle table. In cases that a single data type mapping is not clear, alternative data type mappings are pr...