--然后测试 ON UPDATE SET DEFAULT--删除前面的外键约束.ALTERTABLEtest_sub_studentDROPCONSTRAINTfk_main_class;--创建外键约束.ALTERTABLEtest_sub_studentADDCONSTRAINTfk_main_classFOREIGNKEY(main_id)REFERENCEStest_main_classONUPDATESETDEFAULT;UPDATEtest_main_classSETID=20WHEREID=2;SELECTtest_main_class.valu...
可使用 INSERT INTO 语句向空表写入数据。 SQL 约束 约束用于限制加入表的数据的类型。 可以在创建表时规定约束(通过 CREATE TABLE 语句),或者在表创建之后也可以(通过 ALTER TABLE 语句)。 我们将主要探讨以下几种约束: NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT SQL NOT NULL 约束 NOT NULL 约束...
为了更精确的查询出特定数据,我们需要学习一个新的SQL语法:SELECT查询的 WHERE 子句. 一个查询的 WHERE子句用来描述哪些行应该进入结果,具体就是通过 condition条件 限定这些行的属性满足某些具体条件。比如:WHERE 体重大于 10KG的狗。你可以把 WHERE想象成一个 筛子,每一个特定的筛子都可以筛下某些豆子。
SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the act...
SQL UNIQUE Constraint on CREATE TABLE 下面的 SQL 在“Persons” 表创建时在 “Id_P” 列创建 UNIQUE 约束: MySQL: CREATETABLEPersons(Id_PintNOTNULL,LastNamevarchar(255)NOTNULL,FirstNamevarchar(255),Addressvarchar(255),Cityvarchar(255),UNIQUE(Id_P)) ...
Write a SQL query to create a table with three columns: id, name, and age. Add a primary key constraint on the id column. Write a SQL query to create a table with four columns: product_id, product_name, price, and quantity. Ensure that the price column cannot have negative values...
Explore all SQL constraints with examples and real-world use cases. Learn PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and more.
SQL ConstraintsIn this tutorial you will learn how to use SQL constraints.What is Constraint?A constraint is simply a restriction placed on one or more columns of a table to limit the type of values that can be stored in that column. Constraints provide a standard mechanism to maintain the ...
Commonly Used Constraints in Oracle SQL Most commonly used constraints in Oracle SQL Example: Below example rendering names of all the constraints with the following SQL Query: Employees Table: Example: Getting the SELECT CONSTRAINT_NAME, CONSTRAINT_TYPE, TABLE_NAME ...
INTO Persons IN 'Backup.mdb' FROM Persons 类似的拷贝还可以使用:create table back_table as select * from source_table; 6. SQL 约束 约束用于限制加入表的数据的类型。 可以在创建表时规定约束(通过 CREATE TABLE 语句),或者在表创建之后也可以(通过 ALTER TABLE 语句)。