有时在数据库中创建了一个表格之后,我们发现为该表格加上一个索引有助于该表格的查询速度。要透过SQL 达到这个目的,我们先使用ALTER TABLE指令来告诉数据库我们要更改表格的结构,然后使用ADD INDEX指令告诉数据库我们要添加一个索引。 添加索引的语法为:
(1) SQL如果创建时候,不指定类型那么默认是非聚集索引 (2) 聚集索引和非聚集索引都可以有重复记录,唯一索引不能有重复记录。 (3) 主键 默认是加了唯一约束的聚集索引,但是也可以在主键创建时,指定为唯一约束的非聚集索引,因此主键仅仅是默认加了唯一约束的聚集索引,不能说主键就是加了唯一约束的聚集索引 有点拗...
DROP BLOOMFILTER INDEX (Azure Databricks 上的 Delta Lake) FSCK (Azure Databricks 上的 Delta Lake) GENERATE (Azure Databricks 上的 Delta Lake) MERGE INTO (Azure Databricks 上的 Delta Lake) OPTIMIZE (Azure Databricks 上的 Delta Lake) REORG TABLE (Azure Databricks 上的 Delta Lake) ...
百度试题 题目中国大学MOOC: 在ALTER TABLE语句中使用ADD INDEX语句为表增加索引。相关知识点: 试题来源: 解析 对 反馈 收藏
Sch-M lock must wait for all blocking transactions to be completed on this table. During the wait time, the Sch-M lock blocks all other transactions that wait behind this lock when accessing the same table.ONLINEcan't be set toONwhen an index is being created on a local temporary table....
ALTER TABLE modifies a table definition by altering, adding, or dropping columns and constraints. ALTER TABLE also reassigns and rebuilds partitions, or disables and enables constraints and triggers.
CREATE INDEXfor information on the privileges needed to create indexes Syntax alter_table::= Description of the illustration alter_table.gif Note: You must specify some clause aftertable. That is, none of the clauses aftertableare required, but you must specify at least one of them. ...
ALTER TABLE [IF EXISTS] [<schema_name>.]<table_name> ADD COLUMN <new_column_1> <data_type>, ADD COLUMN <new_column_2> <data_type>; Examples -- Add a column named id in the holo_test table. ALTER TABLE IF EXISTS public.holo_test ADD COLUMN id int; Drop columns (in public pr...
alter table E. mp add column addcolumn int; 答案 A相关推荐 1以下语句错误的是( ) A. alter table emp delete column addcolumn; B. alter table emp modify column addcolumn char(10); C. alter table emp change addcolumn addcolumn int; D. alter table E. mp add column addcolumn int; ...
mysql> ALTER TABLE student RENAME TO student_1; 三、主键和索引修改 删除表中主键 ALTER TABLE student DROP PRIMARY KEY; 添加主键 ALTER TABLE student ADD CONSTRAINT PK_STUDENT PRIMARY KEY (id,class); 添加索引 ALTER TABLE student ADD INDEX index_name (name); ...