test=#altertabletbl_foreigndropconstraintfk_tbl_foreign_a_b ;ALTERTABLEtest=#deletefromtbl_foreign;DELETE2test=#insertintotbl_foreign(a,b)values(1,2),(2,2),(1,1);INSERT03test=#insertintotbl_foreign(a)values(3),(4);INSERT02test=#insertintotbl_foreign(c)values(5);INSERT01test=#select*...
CREATE TABLE orders ( order_id integer PRIMARY KEY, product_no integer, quantity integer ); 给订单表增加外键: alter table orders add constraint orders_product_no_fkey foreign key ("product_no") references products("product_no"); 删除外键约束 如果要删除外键约束,则可以执行以下命令: alter table...
The PostgreSQL FOREIGN KEY is a combination of columns with values based on the primary key values from another table. A foreign key constraint, also known as Referential integrity Constraint, specifies that the values of the foreign key correspond to actual values of the primary key in the othe...
ALTER TABLE 从表名 ADD CONSTRAINT 外键约束名 FOREIGN KEY (从表的外键) REFERENCES 主表名 (主表的主键); 1 注意:如果要给一个已存在的表添加 ON DELETE CASCADE 的外键约束,需要如下步骤: 删除已存在的外键约束。 添加一个 ON DELETE CASCADE 的外键约束。 删除外键约束 格式: alter table 从表名 drop ...
Summary: in this tutorial, you will learn about the PostgreSQL foreign key and how to add foreign keys to tables using foreign key constraints. Introduction to PostgreSQL Foreign Key Constraint In PostgreSQL, a foreign key is a column or a group of columns in a table that uniquely identifies ...
"t2_pkey"PRIMARYKEY, btree (a) Foreign-keyconstraints: "t2_b_fkey"FOREIGNKEY(b)REFERENCESt1(a) Disabled internal triggers: "RI_ConstraintTrigger_c_75213"AFTERINSERTONt2FROMt1NOTDEFERRABLE INITIALLY IMMEDIATEFOREACH ROWEXECUTEPROCEDURE"RI_FKey_check_ins"() ...
FOREIGN KEY (customer_id) REFERENCES customers(id); ``` 3、删除外键约束:如果需要删除外键约束,可以使用 ALTER TABLE 命令来删除。 例如,删除 orders 表格中 customer_id 列上的外键约束: ```sql ALTER TABLE orders DROP CONSTRAINT fk_customer; ``` 总的来说,在 PostgreSQL 中使用外键和约束可以帮助确保...
FOREIGN KEY (column2) REFERENCES table2(id) ); ``` 2、如果表已经创建,可以使用ALTER TABLE语句添加外键约束,如下所示: ```sql ALTER TABLE table1 ADD CONSTRAINT fk_column2 FOREIGN KEY (column2) REFERENCES table2(id); ``` 在这两种情况下,外键约束将被添加到指定的列,并指定参考表和列。确保参...
UNIQUE Constraint - 确保值唯一 PRIMARY Key - 数据库唯一主键标识。 FOREIGN Key - 表外键 CHECK Constraint - CHECK约束确保列中的所有值都满足特定条件。 EXCLUSION Constraint - EXCLUDE约束确保运算符在指定的列或表达式上进行比较任何两行,则并非所有这些比较都将返回TRUE。
constraint_type有四种:UNIQUE、PRIMARY KEY、CHECK、FOREIGN KEY 通过修改上边sql语句的table_name和constraint_type来进行相应的查询 警告