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Questions and Answers
What SQL statement is used to remove a column from a table?
How can a table be deleted in SQL?
What is the purpose of setting constraints in SQL tables?
What does the PRIMARY KEY constraint ensure in a table?
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When creating tables, what does the FOREIGN KEY constraint establish?
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What SQL statement is used to create a new table in a Database Management System (DBMS)?
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In the SQL code snippet provided, what is the data type of the name column in the customers table?
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What SQL statement should be used to add a new column named phone to the customers table?
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Which clause in the ALTER TABLE statement is used to modify the properties of an existing column, such as changing the data type or adding constraints?
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What does the NOT NULL constraint signify when modifying a table's column in a Database Management System?
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Study Notes
Creating, Modifying, and Deleting Tables in a Database Management System (DBMS): DDL Deep Dive
When building a database to store information, tables are the foundational building blocks. The Database Definition Language (DDL) is a set of instructions used to create, modify, and delete tables in a Database Management System (DBMS). Let's explore these essential DDL concepts in more detail.
Creating Tables
To create a table, you use the CREATE TABLE statement, specifying its name, columns, and data types:
CREATE TABLE customers
(
id INTEGER PRIMARY KEY,
name VARCHAR(50) NOT NULL,
email VARCHAR(100)
);
This code snippet creates a customers table with three columns: id (an integer primary key), name (a string with a length of 50 characters), and email (a string with a length of 100 characters).
Modifying Tables
Sometimes you need to modify existing tables to add or change columns. To do this, you use the ALTER TABLE statement:
ALTER TABLE customers
ADD COLUMN address VARCHAR(100);
This statement adds a new column called address (a string with a length of 100 characters) to the customers table.
To rename a column, you can use the ALTER TABLE statement with the ALTER COLUMN clause:
ALTER TABLE customers
ALTER COLUMN email VARCHAR(255) NOT NULL;
This statement changes the data type of the email column to VARCHAR(255) and adds the NOT NULL constraint, which means the column cannot contain null values.
To delete a column, you use the ALTER TABLE statement with the DROP COLUMN clause:
ALTER TABLE customers
DROP COLUMN address;
This statement removes the address column from the customers table.
Deleting Tables
When a table is no longer needed, it can be deleted using the DROP TABLE statement:
DROP TABLE customers;
This statement removes the customers table from the DBMS.
Setting Constraints
Constraints are used to enforce data integrity and ensure the quality of data in a table. Constraints like primary keys, foreign keys, and not null can be declared during table creation or added later using the ALTER TABLE statement:
CREATE TABLE customers
(
id INTEGER PRIMARY KEY,
name VARCHAR(50) NOT NULL,
email VARCHAR(100) NOT NULL,
FOREIGN KEY (email) REFERENCES users(email)
);
This statement creates a customers table with a foreign key constraint on the email column, which references the primary key of another table called users.
Understanding and applying these DDL concepts are fundamental to building and maintaining a well-structured database. While this article touches upon the basics, DDL can be more complex in reality, and further exploration of advanced features and tools is encouraged.
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Description
Learn about creating, modifying, and deleting tables in a Database Management System (DBMS) using the Database Definition Language (DDL). Explore how to set constraints like primary keys and foreign keys for maintaining data integrity. Dive deep into essential DDL concepts for building and managing a well-structured database.
