In most databases, there will be dependencies between objects in the database. Sometimes, we need to understand these dependencies to figure out how to perform certain actions, such as modifying or deleting existing objects. Let's look at this in detail.
We'll use the following simple database to understand and investigate them:
- Create two tables as follows:
CREATE TABLE orders ( orderid integer PRIMARY KEY ); CREATE TABLE orderlines ( orderid integer ,lineid smallint ,PRIMARY KEY (orderid, lineid) );
ALTER TABLE orderlines ADD FOREIGN KEY (orderid) REFERENCES orders (orderid);
- If we try to drop the referenced table, we get the following message:
DROP TABLE orders; ERROR: cannot drop table orders because other objects depend on it DETAIL: constraint orderlines_orderid_fkey on table orderlines depends on table orders HINT: Use DROP ... CASCADE to drop the dependent...