Null values and SQL contraints

The rules on the columns of a table are restrictions. These are used to limit the data type entered in a table. This ensures that the data in the database are accurate and reliable.

The limitations might either be on a column or a table level. The column-level constraints are applied to a single column, whereas the column-level limit is applied to the entire table.

Some of the most often used limitations in SQL are given below. These limitations have been mentioned in the SQL - RDBMS chapter. However, at this point, it is worth reviewing them.

  • NOT NULL Location − Makes sure a column does not have a value of NULL.
  • Default restriction - Provides a default column value when no one is defined. • Default constraint.
  • UNIQUE Limit − Checks to ensure that every column value is different.
  • PRIMARY Key − Using a database table, you may identify every row/cord.
  • Forge Key − Unique in any particular database table is to identify a row/record.
  • CHECK Constraint − The CHECK restriction guarantees that all the column values fulfil specific requirements.
  • INDEX − The database is used for very speedy data creation and retrieval.

The limitations can be defined if a table is created using the CREATE TABLE statement or when the table is formed. You can use the ALTER TABLE statement to create restrictions.

Dropping Constraints

The Altar Table command with the DROP CONSTRAINT argument allows you to remove any restriction you have established.

Some systems allow restrictions to be disabled. You may want to temporarily disable the limit rather than permanently drop a rule from the database and then allow it after that.

Integrity Constraints

Integrity restrictions are employed to ensure that the data in a relational database are accurate and consistent. Data integrity is managed through referential integrity in a relational database.

There are numerous types of integrity constraints (RI). These limitations include primary key, foreign key, single restrictions, and other limitations described earlier.

NULL Values

The word SQL NULL represents a missing value, and a table NULL value is a field value that seems blank.

A NULL value field is a value-free field, and it is crucial to realise that a NULL value differs from a zero value or a space field.

NOT NULL here means that the column should always take an explicit data type value. There are two columns in which NOT NULL has not been used, representing these columns could be NULL.

The one left blank during record generation is a field with NULL values.