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Questions and Answers
What type of constraint is applied to a single column only?
What type of constraint is applied to a single column only?
Which constraint ensures that a value cannot be NULL?
Which constraint ensures that a value cannot be NULL?
In database terminology, what does UNIQUE constraint prevent?
In database terminology, what does UNIQUE constraint prevent?
What does a Table Level Constraint affect?
What does a Table Level Constraint affect?
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How many primary keys can a table have?
How many primary keys can a table have?
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What does a PRIMARY KEY consist of in a database table?
What does a PRIMARY KEY consist of in a database table?
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Which constraint prevents the insertion of NULL values but allows duplicate values?
Which constraint prevents the insertion of NULL values but allows duplicate values?
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What happens if a column has a DEFAULT constraint when no value is specified during INSERT?
What happens if a column has a DEFAULT constraint when no value is specified during INSERT?
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What does the CHECK constraint allow you to do in a database table?
What does the CHECK constraint allow you to do in a database table?
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In a composite key, what is the significance of having more than one column as part of the key?
In a composite key, what is the significance of having more than one column as part of the key?
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Study Notes
Column Constraints
- A column-level constraint is applied specifically to a single column in a database table.
NOT NULL Constraint
- The NOT NULL constraint ensures that a specific value within a column cannot be NULL, enforcing data integrity.
UNIQUE Constraint
- The UNIQUE constraint prevents duplicate values in a column, ensuring that each entry remains distinct across the dataset.
Table Level Constraints
- A table-level constraint applies to the entire table instead of individual columns, affecting multiple columns or the table as a whole.
Primary Keys
- A table can have only one primary key, which uniquely identifies each record within the table.
Components of a Primary Key
- A PRIMARY KEY consists of one or more columns that must have unique values for each row, ensuring no two records are the same.
NULL Value Prevention
- The UNIQUE constraint does not prevent NULL values and allows duplicates as long as the non-null entries are unique.
DEFAULT Constraint Behavior
- If a column includes a DEFAULT constraint, it automatically assigns a specified value to that column when no value is provided during an INSERT operation.
CHECK Constraint
- The CHECK constraint allows for the definition of specific conditions for a column's values, ensuring that only valid data is entered into the database.
Significance of Composite Keys
- In a composite key, having multiple columns as part of the key ensures a unique combination of values, enhancing record identification beyond single columns.
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Description
Learn about the various data constraints that can be enforced in a database table, including primary key, not null, unique, default, check, and foreign key constraints. Understand the classification of constraints as column level and table level constraints.
