![]() If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s). When multiple fields are used as a primary key, they are called a composite key. A primary key column cannot have NULL values.Ī table can have only one primary key, which may consist of single or multiple fields. This is not the case with other databasesĪ primary key is a field in a table, which uniquely identifies each row/record in a database table. ![]() Due to a 'longstanding coding oversight', primary keys can be NULL in SQLite. Primary keys become foreign keys in other tables, when creating relations among tables. Primary keys are important when designing the database tables. There can be more UNIQUE columns, but only one primary key in a table. The PRIMARY KEY constraint uniquely identifies each record in a database table. Here, AGE column is set to UNIQUE, so that you cannot have two records with same age − Exampleįor example, the following PostgreSQL statement creates a new table called COMPANY3 and adds five columns. In the COMPANY table, for example, you might want to prevent two or more people from having identical age. The UNIQUE Constraint prevents two records from having identical values in a particular column. Exampleįor example, the following PostgreSQL statement creates a new table called COMPANY1 and adds five columns, three of which, ID and NAME and AGE, specify not to accept NULL values − A NOT NULL constraint is always written as a column constraint.Ī NULL is not the same as no data rather, it represents unknown data. ![]() If you do not want a column to have a NULL value, then you need to define such constraint on this column specifying that NULL is now not allowed for that column. PRIMARY Key − Uniquely identifies each row/record in a database table.įOREIGN Key − Constrains data based on columns in other tables.ĬHECK Constraint − The CHECK constraint ensures that all values in a column satisfy certain conditions.ĮXCLUSION Constraint − The EXCLUDE constraint ensures that if any two rows are compared on the specified column(s) or expression(s) using the specified operator(s), not all of these comparisons will return TRUE.īy default, a column can hold NULL values. UNIQUE Constraint − Ensures that all values in a column are different. NOT NULL Constraint − Ensures that a column cannot have NULL value. The following are commonly used constraints available in PostgreSQL. For example, a column of type DATE constrains the column to valid dates. Defining a data type for a column is a constraint in itself. Column level constraints are applied only to one column whereas table level constraints are applied to the whole table. This ensures the accuracy and reliability of the data in the database.Ĭonstraints could be column level or table level. These are used to prevent invalid data from being entered into the database. Constraints are the rules enforced on data columns on table.
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