ALTER TABLE doctors ADD CONSTRAINT chk_specialization CHECK (specialization IN ('Pediatrics', 'Internal Medicine', 'Surgery')); NOT NULL 约束: 如上所述,name 列在两个表中都不应该为空,因为每个医生和患者都应该有一个名字。 结论 上述场景展示了如何在实际的医院环境中使用SQL的 CONSTRAINT 来确保数据的...
ContactName varchar(255), Country varchar(255), PRIMARY KEY (CustomerID) ); 四、CONSTRAINTS AND DATABASE INTEGRITY Constraints play a crucial role in maintaining the integrity and consistency of data in a database. For example, the PRIMARY KEY and FOREIGN KEY constraints prevent duplicate and NU...
mysql> insert into t_user(id,name,email) values(2,'mmm','qq.com'); Query OK, 1 row affected (0.05 sec) 插入第三条数据,如果与联合字段都相同,则报错 mysql> insert into t_user(id,name,email) values(3,'mmm','qq.com'); ERROR 1062 (23000): Duplicate entry 'mmm-qq.com' for key ...
SQL database in Microsoft Fabric Specifies the properties of a PRIMARY KEY, UNIQUE, FOREIGN KEY, a CHECK constraint, or a DEFAULT definition added to a table by usingALTER TABLE. Transact-SQL syntax conventions Syntax syntaxsqlCopy [CONSTRAINTconstraint_name] { {PRIMARYKEY|UNIQUE} [CLUSTERED|NON...
The general formula for adding check constraint in SQL Server: ALTER TABLE { TABLE_NAME } ADD CONSTRAINT { CONSTRAINT_NAME } CHECK ( BOOLEAN_EXPRESSION ) If the BOOLEAN_EXPRESSION returns true, then the CHECK constraint allows the value, otherwise it doesn't. Since, AGE is a nullable column...
CREATE TABLE students ( id INT PRIMARY KEY, name VARCHAR(100), age INT CHECK (age >= 0 AND age <= 100) ); 通过触发器实现检查约束: 代码语言:txt 复制 DELIMITER $$ CREATE TRIGGER check_age_before_insert BEFORE INSERT ON students FOR EACH ROW BEGIN IF NEW.age < 0 OR NEW.age > 100...
FirstName varchar(255), Age int, UNIQUE(ID) ); To name aUNIQUEconstraint, and to define aUNIQUEconstraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATETABLEPersons ( ID intNOTNULL, ...
CREATE TABLE users ( id INT PRIMARY KEY, name VARCHAR(255), department_id INT, FOREIGN KEY (department_id) REFERENCES departments(id) ); 如果你决定删除这个外键约束,可以使用以下SQL语句: 代码语言:txt 复制 ALTER TABLE users DROP FOREIGN KEY fk_department; ...
Syntax for SQL Server, Azure SQL Database, Azure SQL Managed Instance, SQL database in Microsoft Fabric syntaxsql [CONSTRAINTconstraint_name] { [NULL|NOTNULL] {PRIMARYKEY|UNIQUE} [CLUSTERED|NONCLUSTERED] [WITHFILLFACTOR= fillfactor ] [WITH(index_option[, ...n ] ) ] [ON{partition_scheme_na...
Namespace: Java.Sql Assembly: Mono.Android.dll The subclass ofSQLExceptionthrown when the SQLState class value is '23', or under vendor-specified conditions. C#复制 [Android.Runtime.Register("java/sql/SQLIntegrityConstraintViolationException", DoNotGenerateAcw=true)]publicclassSQLIntegrityConstraintViol...