候選索引 (藉由包含 UNIQUE 選項所建立,為 ALTER TABLE 或 CREATE TABLE) 提供的 ANSI 相容性提供,與使用 INDEX 命令中的 UNIQUE 選項所建立的索引不同。 在 INDEX 命令中使用 UNIQUE 建立的索引允許重複的索引鍵;候選索引不允許重複的索引鍵。 在用於主要或候選索引的欄位中,不允許 Null 值和重複的記錄。
The ALTER TABLE command allows you to add, modify, or drop a column from an existing table. Adding column(s) to a table Syntax #1 To add a column to an existing table, the ALTER TABLE syntax is: ALTER TABLE table_name ADD column_name column-definition; For example: ALTER TABLE supplie...
一、语法定义 还是参考官方文档,其定义如下: 从这个定义上我们可以看到,必要的语句是 altertabletbl_name[command]. 同时,这条语句还支持多个命令,相信很多人在修改一个表,比如增加几列时,都会写多个alter语句,但实际上一条就搞定了。 二、语义详解: 下面我们对add相关的语法进行详细分析 1. ADD [COLUMN]col_na...
ALTER TABLE TableName1 ADD | ALTER [COLUMN] FieldName1 FieldType [(nFieldWidth [, nPrecision])] [NULL | NOT NULL] [CHECK lExpression1 [ERROR cMessageText1]] [DEFAULT eExpression1] [PRIMARY KEY | UNIQUE] [REFERENCES TableName2 [TAG TagName1]] [NOCPTRANS] - Or - ALTER TABLE Table...
SQL ALTER TABLE 命令用于添加、删除或者更改现有数据表中的列。你还可以用 ALTER TABLE 命令来添加或者删除现有数据表上的约束。语法:使用ALTER TABLE 在现有的数据表中添加新列的基本语法如下:ALTER TABLE table_name ADD column_name datatype;使用ALTER TABLE 在现有的数据表中删除列的基本语法如下:...
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.
I am having a problem when I run an alter command using MYSQL. I get the following error message, "Can't create table '.\flybynight\#sql-6f8_9.frm'". I get this error message when I attempt to run an alter table command on a table in my flybynight database. If anyone knows ...
Which can make your SQL that bit faster.You can also quickly load rows into a partition from another table with partition exchange. But to use this the two tables must have identical structures. Which can be hard to validate.So Oracle Database 12.2 introduced a new create table option. ...
The ALTER TABLE command adds, deletes, or modifies columns in a table.The ALTER TABLE command also adds and deletes various constraints in a table.The following SQL adds an "Email" column to the "Customers" table:ExampleGet your own SQL Server ALTER TABLE CustomersADD Email varchar(255); ...
TheCREATE TABLEcommand creates a new table in the database. The following SQL creates a table called "Persons" that contains five columns: PersonID, LastName, FirstName, Address, and City: Example CREATETABLEPersons ( PersonID int, LastName varchar(255), ...