SQL Constraints (2024)

FAQs

What are the 5 constraints in SQL? ›

To find the name of a constraint, you can use the following query: SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA. TABLE_CONSTRAINTS WHERE TABLE_NAME = 'table_name' AND CONSTRAINT_TYPE = 'constraint_type';

ADD CONSTRAINT is a SQL command that is used together with ALTER TABLE to add constraints (such as a primary key or foreign key) to an existing table in a SQL database. The basic syntax of ADD CONSTRAINT is: ALTER TABLE table_name ADD CONSTRAINT PRIMARY KEY (col1, col2);

A unique constraint is the rule that the values of a key are valid only if they are unique. A key that is constrained to have unique values is called a unique key . A unique constraint is enforced by using a unique index.

Every project has to manage four basic constraints: scope, schedule, budget and quality. The success of a project depends on the skills and knowledge of the project manager to take into consideration all these constraints and develop the plans and processes to keep them in balance.

SELECT CONSTRAINT_NAME, CONSTRAINT_TYPE FROM INFORMATION_SCHEMA. TABLE_CONSTRAINTS WHERE TABLE_NAME='student_info'; Note: here, three rows will be displayed since the table has 3 constraints which are the primary key, the check constraint, and the unique constraint.

SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.

The primary key in SQL is a single, or a group of fields or columns that can uniquely identify a row in a table. Putting it simply, it is a column that accepts unique values for each row.

Normalization is the process of organizing data in a database. It includes creating tables and establishing relationships between those tables according to rules designed both to protect the data and to make the database more flexible by eliminating redundancy and inconsistent dependency.

What is schema in SQL? ›

What is Schema in SQL? In a SQL database, a schema is a list of logical structures of data. A database user owns the schema, which has the same name as the database manager. As of SQL Server 2005, a schema is an individual entity (container of objects) distinct from the user who constructs the object.

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 column it will allow only certain values for this column.

The NOT NULL constraint enforces a column to not accept NULL values, which means that you cannot insert or update a record without adding a value to this field.

The DEFAULT constraint is used to set a default value for a column. The default value will be added to all new records, if no other value is specified.

Primary key constraints

NULL values are not allowed. If the column(s) contain NULL values, the system will not add the primary key constraint. See ALTER TABLE statement for more information. A table can have at most one PRIMARY KEY constraint.

Constraints are things that limit or put boundaries around your projects. Scope, schedule, and cost are the big three, but risk, quality, resources, regulations, and other factors are also relevant,” explains Alan Zucker, Founder of Project Management Essentials, LLC.

There are six main constraints that are commonly used in SQL Server that we will describe deeply with examples within this article and the next one.

The three primary constraints that project managers should be familiar with are time, scope, and cost. These are frequently referred to as the triple constraints or the project management triangle.