test=#altertabletbl_uniquedropconstraintuk_tbl_unique_a_b ;ALTERTABLEtest=#deletefromtbl_unique ;DELETE0test=#insertintotbl_unique (a,b)values(1,1),(1,1),(1,1);INSERT03test=#insertintotbl_unique (a)values(2),(2),(2);INSERT03test=#selectoid,*fromtbl_unique ; oid|a|b|c---+-...
The UNIQUE constraint ensures that all values in a column are unique. This tutorial covers how to use the UNIQUE constraint with practical examples. Setting Up the DatabaseFirst, let's create the users table with a UNIQUE constraint on the email column. ...
CREATE UNIQUE NONCLUSTERED INDEX [IX_TestUnique_SiteIdUrl] ON [TestUnique] (SiteId,Url) 1. 2. 3. 消息1908,级别16,状态1,第1 行 列'Id' 是索引'IX_TestUnique_SiteIdUrl' 的分区依据列。唯一索引的分区依据列必须是索引键的子集。 --方式2 ALTER TABLE [dbo].[TestUnique] ADD CONSTRAINT [IX...
);altertable"SysUser"addconstraintPK_SysUserprimarykey("UserId");--说明:通过修改表结构设置主键,可以设置一列或多列作为主键,可以指定主键名称。 4.往已有表添加自增主键 --创建没有任何主键的表。createtable"Vendors" ("Name"varchar(255));--往表添加数据insertinto"Vendors"("Name")values('001'),(...
and table_constraint is: [ CONSTRAINT constraint_name ] { CHECK ( expression ) [ NO INHERIT ] | UNIQUE [ NULLS [ NOT ] DISTINCT ] ( column_name [, ... ] ) index_parameters | PRIMARY KEY ( column_name [, ... ] ) index_parameters | ...
The unique constraint in PostgreSQL ensures that the uniqueness of the values entered into a column or a field of a table.
UNIQUE 约束 UNIQUE 约束可以设置列是唯一的,避免同一列出现重复值。 实例 下面实例创建了一张新表叫 COMPANY3,添加了 5 个字段,其中 AGE 设置为 UNIQUE,因此你不能添加两条有相同年龄的记录: CREATE TABLE COMPANY3( ID INT PRIMARY KEY NOT NULL, ...
Postgres enables us to implement the UNIQUE constraint on more than one column using the following syntax: CREATE TABLE tab_name ( col_1 data_type, col_2 data_type, …, col_n data_type, UNIQUE (col_1, col_2)); The following syntax will work the same way as the above syntax: ...
唯一约束 唯一约束可以用两种方式定义:列级唯一约束和表级唯一约束 列级唯一约束演示: create table student( student_id...primary key, student_name varchar(30) not null, email varchar(30) unique, student_age tinyint(3)); 表级唯一约束演示...constraint student_email_uk unique(email) );//表级...
PostgreSQL allows you to create a UNIQUE constraint to a group of columns using the following syntax: CREATE TABLE table ( c1 data_type, c2 data_type, c3 data_type, UNIQUE (c2, c3) ); The combination of values in the columns c2 and c3 will be unique across the whole table. The valu...