This keeps us from duplicating phone numbers. The following SQL covers both PRIMARY KEY and FOREGIN KEY, which we will discuss next.įOREIGN KEY (“friend”) REFERENCES “friends”(“id”)) Ībove we define “number” as the PRIMARY KEY for the table. Just like UNIQUE, you can combine fields to create a PRIMARY KEY. Unlike UNIQUE, a PRIMARY KEY cannot contain NULL. While you can have many UNIQUE constraints, you can have only one PRIMARY KEY. The PRIMARY KEY also uniquely identifies a record, but also acts as the identity of the record. Just use the format “CONSTRAINT “constraint_name” ()”. This naming convention works for all of the constraints. You can also name the constraint using the following format.ĬONSTRAINT “uc_nickname” UNIQUE (“Nickname”)) “id” INT GENERATED BY DEFAULT AS IDENTITY, If we created a table of friends and we wanted to make sure that the nickname we give each friend is unique, we can create a UNIQUE constraint when creating the table. You can have many UNIQUE constraints in a table, and you can combine fields to create a UNIQUE constraint. UNIQUE uniquely defines a record in a table.
In this post we will look at four more, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK. We have already discussed two of the constraints, NOT NULL and DEFAULT. Constraints specify rules for the data in a table’s fields.
0 kommentar(er)
