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. ...
);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'),(...
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...
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...
The unique constraint in PostgreSQL ensures that the uniqueness of the values entered into a column or a field of a table.
create table table_name ( column_name type column_constraint, table_constraint table_constraint ) inherits existing_table_name; 1. 2. 3. 4. 示例: create table account( user_id serial primary key, username varchar(50) unique not null, ...
Postgres enables us to implement the UNIQUE constraint on more than one column using the following syntax: CREATETABLE tab_name (col_1data_type,col_2data_type,…,col_ndata_type,UNIQUE(col_1, col_2)); The following syntax will work the same way as the above syntax: ...
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, ...
This method can also be used to create a constraint that allows only a single null for each non-null composite indexed value: CREATE TABLE nulltest2 (x INTEGER, y INTEGER);CREATE UNIQUE INDEX i_nulltest2 ON nulltest2 (x, (y IS NULL)) WHEREy IS NULL;INSERT INTO nulltest2 VALUES (...