90 lines
1.9 KiB
Markdown
90 lines
1.9 KiB
Markdown
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---
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title: SQL CHECK Constraint
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localeTitle: SQL CHECK约束
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---
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CHECK约束用于限制可以放在列中的值范围。
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如果在单个列上定义CHECK约束，则它仅允许此列的某些值。
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如果在表上定义CHECK约束，它可以根据行中其他列中的值限制某些列中的值。
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### SQL CHECK CREATE TABLE
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创建“Persons”表时，以下SQL在“Age”列上创建CHECK约束。 CHECK约束确保您不能拥有18岁以下的任何人：
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**MySQL的：**
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```sql
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CREATE TABLE Persons (
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ID int NOT NULL,
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LastName varchar(255) NOT NULL,
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FirstName varchar(255),
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Age int,
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CHECK (Age>=18)
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);
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```
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**SQL Server / Oracle / MS Access：**
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```sql
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CREATE TABLE Persons (
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ID int NOT NULL,
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LastName varchar(255) NOT NULL,
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FirstName varchar(255),
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Age int CHECK (Age>=18)
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);
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```
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要允许命名CHECK约束，并在多列上定义CHECK约束，请使用以下SQL语法：
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**MySQL / SQL Server / Oracle / MS Access：**
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```sql
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CREATE TABLE Persons (
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ID int NOT NULL,
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LastName varchar(255) NOT NULL,
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FirstName varchar(255),
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Age int,
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City varchar(255),
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CONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes')
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);
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```
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### SQL CHECK ALTER TABLE
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要在已创建表时在“Age”列上创建CHECK约束，请使用以下SQL：
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**MySQL / SQL Server / Oracle / MS Access：**
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```sql
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ALTER TABLE Persons
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ADD CHECK (Age>=18);
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```
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要允许命名CHECK约束，并在多列上定义CHECK约束，请使用以下SQL语法：
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**MySQL / SQL Server / Oracle / MS Access：**
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```sql
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ALTER TABLE Persons
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ADD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City='Sandnes');
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```
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### DROP一个CHECK约束
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要删除CHECK约束，请使用以下SQL：
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**SQL Server / Oracle / MS Access：**
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```sql
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ALTER TABLE Persons
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DROP CONSTRAINT CHK_PersonAge;
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```
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**MySQL的：**
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```sql
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ALTER TABLE Persons
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DROP CHECK CHK_PersonAge;
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```
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