The NOT NULL constraint enforces a column to NOT accept NULL values. The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field. The following...
SQL UNIQUE Constraint The UNIQUE constraint uniquely identifies each record in a database table. The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has...
SQL PRIMARY KEY Constraint The PRIMARY KEY constraint uniquely identifies each record in a database table. Primary keys must contain UNIQUE values. A primary key column cannot contain NULL values. Most tables should have a primary key, and each table...
SQL FOREIGN KEY Constraint A FOREIGN KEY in one table points to a PRIMARY KEY in another table. CREATE TABLE Orders ( O_Id int NOT NULL PRIMARY KEY, OrderNo int NOT NULL, P_Id int FOREIGN KEY REFERENCES Persons(P_Id) ) To DROP a FOREIGN KEY Constraint...
SQL Constraints SQL constraints are used to specify rules for the data in a table. If there is any violation between the constraint and the data action, the action is aborted by the constraint. Constraints can be specified when the table is created (inside...
SQL CHECK Constraint The CHECK constraint is used to limit the value range that can be placed in a column. If you define a CHECK constraint on a single column it allows only certain values for this column. If you define a CHECK constraint on a table it...
SQL Scalar functions SQL scalar functions return a single value, based on the input value. Useful scalar functions: UCASE() - Converts a field to upper case LCASE() - Converts a field to lower case MID() - Extract characters from a text field LEN() -...
SQL Aggregate Functions SQL aggregate functions return a single value, calculated from values in a column. Useful aggregate functions: AVG() - Returns the average value COUNT() - Returns the number of rows FIRST() - Returns the first value LAST() - Returns...
