Constraints

Constraints in SQL are the rules applied to the values of individual columns. They add information about how a column can be used after specifying the data type for a column. They can be used to tell the database to reject inserted data that does not adhere to a certain restriction.
Here are some of the constraints that can be set:
PRIMARY KEY
columns can be used to uniquely identify the row. Attempts to insert a row with an identical value to a row already in the table will result in a constraint violation which will not allow you to insert the new row.UNIQUE
columns have a different value for every row. This is similar toPRIMARY KEY
except a table can have many differentUNIQUE
columns.NOT NULL
columns must have a value. Attempts to insert a row without a value for aNOT NULL
column will result in a constraint violation and the new row will not be inserted.DEFAULT
columns take an additional argument that will be the assumed value for an inserted row if the new row does not specify a value for that column.
Note: There can be only one PRIMARY KEY
column per table, but there can be multiple UNIQUE
columns.
The statement below sets constraints on the celebs
table:
CREATE TABLE celebs (id INTEGER PRIMARY KEY,name TEXT UNIQUE,grade INTEGER NOT NULL,age INTEGER DEFAULT 10);
id
column is thePRIMARY KEY
.name
column isUNIQUE
.grade
column isNOT NULL
.age
column has aDEFAULT
of 10.
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