83 lines
2.0 KiB
Markdown
83 lines
2.0 KiB
Markdown
---
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title: SQL CHECK Constraint
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---
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The CHECK constraint is used to limit the value range that can be placed in a column.
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If you define a CHECK constraint on a single column it allows only certain values for this column.
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If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.
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### SQL CHECK on CREATE TABLE
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The following SQL creates a CHECK constraint on the "Age" column when the "Persons" table is created. The CHECK constraint ensures that you can not have any person below 18 years:
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**MySQL:**
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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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To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
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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 on ALTER TABLE
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To create a CHECK constraint on the "Age" column when the table is already created, use the following 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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To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
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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 a CHECK Constraint
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To drop a CHECK constraint, use the following 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:**
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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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