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SQL - ALTER TABLE

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Check out all of the SQL Keywords
ADD
ADD CONSTRAINT
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ALTER
The ALTER keyword is used to modify the structure of an existing database object, such as a table. It allows users to add, delete, or change columns and constraints within the table.
ALTER COLUMN
The ALTER COLUMN keyword is used to modify the definition of an existing column in a table, such as changing its data type, size, or constraints.
ALTER TABLE
The ALTER TABLE keyword is used to modify an existing table structure in a database. This can include adding, deleting, or modifying columns, as well as changing constraints or other properties of the table.
AND
The AND keyword is used in SQL to combine multiple conditions in a WHERE clause, ensuring that all specified conditions must be true for a record to be included in the result set.
ANY
The ANY keyword is used in SQL to compare a value to any value in a list or a subquery result. It allows for flexible comparisons, enabling the retrieval of records that meet certain criteria based on multiple potential matches.
AS
The AS keyword is used to create an alias for a table or a column in a SQL query, allowing for easier reference and improved readability in the results.
ASC
The ASC keyword is used to specify the sorting order of the results in ascending order when retrieving data from a database.
BACKUP DATABASE
The BACKUP DATABASE keyword is used to create a copy of a database, preserving its data and structure, which can be used for recovery purposes in case of data loss or corruption.
BETWEEN
The BETWEEN keyword is used in SQL to filter the result set within a specified range. It can be applied to numeric, date, or text values, allowing users to retrieve records that fall within the defined lower and upper bounds.
CASE
The CASE keyword is used to create conditional logic within SQL statements, allowing for the evaluation of conditions and the return of specific values based on those conditions.
CHECK
The CHECK keyword is used to impose a condition on the values that can be inserted or updated in a column of a table, ensuring that only data that meets the specified criteria is allowed.
COLUMN
The COLUMN keyword is used in SQL to specify a particular column in a table, often in conjunction with other commands to define, modify, or retrieve data related to that column.
CONSTRAINT
The CONSTRAINT keyword is used to define rules or limitations on the data in a table, ensuring data integrity and enforcing specific conditions on the values that can be stored in the table's columns.
CREATE
The CREATE keyword is used to create new database objects, such as tables, views, indexes, or databases, defining their structure and properties.
CREATE DATABASE
The CREATE DATABASE keyword is used to create a new database in a SQL server, allowing users to store and manage data within that database.
CREATE INDEX
The CREATE INDEX keyword is used to create an index on one or more columns of a table, which improves the speed of data retrieval operations by allowing the database to find rows more efficiently.
CREATE OR REPLACE VIEW
The CREATE OR REPLACE VIEW keyword is used to create a new view or update an existing view in a database. A view is a virtual table that provides a way to present data from one or more tables in a specific format or structure without storing the data physically.
CREATE PROCEDURE
The CREATE PROCEDURE keyword is used to define a new stored procedure in a database. A stored procedure is a set of SQL statements that can be executed as a single unit, allowing for reusable and modular code.
CREATE TABLE
The CREATE TABLE keyword is used to create a new table in a database, defining its structure by specifying the table name and the columns along with their data types and constraints.
CREATE UNIQUE INDEX
The CREATE UNIQUE INDEX keyword is used to create a unique index on one or more columns of a table, ensuring that the values in those columns are distinct across all rows in the table. This helps maintain data integrity by preventing duplicate entries in the specified columns.
CREATE VIEW
The CREATE VIEW keyword is used to define a virtual table based on the result set of a SELECT query. This allows users to simplify complex queries, encapsulate logic, and present data in a specific format without storing the data physically.
DATABASE
The DATABASE keyword is used to create, select, or manage a database within a database management system, allowing users to organize and store data efficiently.
DEFAULT
The DEFAULT keyword is used to specify a default value for a column in a table when no value is provided during an insert operation.
DELETE
The DELETE keyword is used to remove existing records from a table based on specified conditions.
DESC
The DESC keyword is used to sort the result set of a query in descending order, typically applied to a specific column in a SELECT statement.
DISTINCT
The DISTINCT keyword is used in SQL to eliminate duplicate rows from the result set of a query, ensuring that only unique values are returned.
DROP
The DROP keyword is used to remove an entire table, view, index, or database from the database management system, permanently deleting all associated data and structure.
DROP COLUMN
The DROP COLUMN keyword is used to remove an existing column from a table in a database.
DROP CONSTRAINT
The DROP CONSTRAINT keyword is used to remove a constraint from a table, which can include primary keys, foreign keys, unique constraints, or check constraints, thereby altering the rules that govern the data within that table.
DROP DATABASE
The DROP DATABASE keyword is used to permanently remove an entire database and all of its associated data, tables, and objects from the database management system.
DROP DEFAULT
The DROP DEFAULT keyword is used to remove the default value assigned to a column in a table. This means that if no value is provided for that column during an insert operation, the column will no longer automatically receive the previously defined default value.
DROP INDEX
The DROP INDEX keyword is used to remove an existing index from a database table, which can help improve performance by reducing the overhead associated with maintaining the index during data modifications.
DROP TABLE
The DROP TABLE keyword is used to delete an entire table and all of its data from a database permanently.
DROP VIEW
The DROP VIEW keyword is used to remove an existing view from the database.
EXEC
The EXEC keyword is used to execute a stored procedure or a dynamic SQL statement in a database management system.
EXISTS
The EXISTS keyword is used in SQL to test for the existence of any records in a subquery. It returns true if the subquery returns one or more rows, and false if it returns no rows.
FOREIGN KEY
The FOREIGN KEY keyword is used to establish a relationship between two tables by linking a column in one table to the primary key of another table, ensuring referential integrity.
FROM
The FROM keyword is used to specify the table or tables from which to retrieve data in a SQL query.
FULL OUTER JOIN
The FULL OUTER JOIN keyword is used to combine rows from two or more tables based on a related column, returning all records from both tables. When there is no match, NULL values are returned for the columns of the table that lacks a corresponding row.
GROUP BY
The GROUP BY keyword is used in SQL to arrange identical data into groups, allowing for the aggregation of values within those groups using functions like COUNT, SUM, AVG, etc.
HAVING
The HAVING keyword is used to filter records that work on summarized group data in SQL, typically in conjunction with the GROUP BY clause.
IN
The IN keyword is used in SQL to specify multiple values in a WHERE clause, allowing for filtering records that match any of the specified values in a list.
INDEX
The INDEX keyword is used to create an index on a database table, which improves the speed of data retrieval operations by providing a quick lookup mechanism for specific columns.
INNER JOIN
The INNER JOIN keyword is used to combine rows from two or more tables based on a related column between them, returning only the rows that have matching values in both tables.
INSERT INTO
The INSERT INTO keyword is used to add new rows of data into a specified table in a database.
INSERT INTO SELECT
The INSERT INTO SELECT keyword is used to insert data into a table by selecting it from another table. This allows for the copying of data from one table to another in a single operation.
IS NOT NULL
The IS NOT NULL keyword is used in SQL to filter query results by excluding rows that contain NULL values in a specified column.
IS NULL
The IS NULL keyword is used in SQL to check whether a specified column or expression has a NULL value, which indicates the absence of any data.
JOIN
The JOIN keyword is used to combine rows from two or more tables based on a related column between them, allowing for the retrieval of data that spans multiple tables in a relational database.
LEFT JOIN
The LEFT JOIN keyword is used to combine rows from two or more tables based on a related column, returning all records from the left table and the matched records from the right table. If there is no match, NULL values are returned for columns from the right table.
LIKE
The LIKE keyword is used in SQL to search for a specified pattern in a column. It allows for wildcard characters, such as '%' for any sequence of characters and '_' for a single character, enabling flexible string matching in queries.
LIMIT
The LIMIT keyword is used to specify the maximum number of records to return in a query result, allowing for the control of the output size and improving performance by restricting the amount of data processed.
NOT
The NOT keyword is used in SQL to negate a condition in a query, often in conjunction with other operators such as WHERE, to filter out records that do not meet specified criteria.
NOT NULL
The NOT NULL keyword is used to specify that a column in a database table cannot contain NULL values, ensuring that every record must have a valid entry for that column.
OR
The OR keyword is used in SQL to combine multiple conditions in a WHERE clause, allowing for the selection of records that meet at least one of the specified conditions.
ORDER BY
The ORDER BY keyword is used to sort the result set of a query by one or more columns in either ascending or descending order.
OUTER JOIN
The OUTER JOIN keyword is used to combine rows from two or more tables based on a related column, including all records from one table and the matched records from the other table, filling in with NULLs where there is no match.
PRIMARY KEY
The PRIMARY KEY keyword is used to uniquely identify each record in a database table, ensuring that no two rows have the same value in the specified column(s) and enforcing entity integrity.
PROCEDURE
The PROCEDURE keyword is used to define a stored procedure in SQL, which is a set of precompiled SQL statements that can be executed as a single unit to perform a specific task or operation within a database.
RIGHT JOIN
The RIGHT JOIN keyword is used to combine rows from two or more tables based on a related column, returning all records from the right table and the matched records from the left table. If there is no match, NULL values are returned for columns from the left table.
ROWNUM
The ROWNUM keyword is used in Oracle SQL to assign a unique sequential integer to each row returned by a query, starting from 1 for the first row. It is often used to limit the number of rows returned in a result set or to filter results based on their position in the output.
SELECT
The SELECT keyword is used to retrieve data from one or more tables in a database. It allows users to specify which columns to display and can include conditions to filter the results.
SELECT DISTINCT
The SELECT DISTINCT keyword is used to retrieve unique values from a specified column or columns in a database table, eliminating duplicate entries from the result set.
SELECT INTO
The SELECT INTO keyword is used to create a new table and populate it with data retrieved from an existing table.
SELECT TOP
The SELECT TOP keyword is used to specify the number of records to return from a query, allowing users to limit the result set to a specified number of rows, often used for performance optimization or to retrieve a sample of data.
SET
The SET keyword is used to assign a value to a variable or to update existing records in a table during an UPDATE statement.
TABLE
The TABLE keyword is used to define a database table structure, specifying the name of the table and its associated columns and data types.
TOP
The TOP keyword is used in SQL to limit the number of rows returned in a query result set, allowing users to specify a specific number or percentage of rows to retrieve from a table.
TRUNCATE TABLE
The TRUNCATE TABLE keyword is used to delete all rows from a table quickly and efficiently, while preserving the table structure for future use.
UNION
The UNION keyword is used to combine the results of two or more SELECT statements into a single result set, eliminating duplicate rows in the process.
UNION ALL
The UNION ALL keyword is used to combine the results of two or more SELECT queries, including all duplicate rows from the combined result set.
UNIQUE
The UNIQUE keyword is used to ensure that all values in a specified column or a group of columns are distinct from one another, preventing duplicate entries in a table.
UPDATE
The UPDATE keyword is used to modify existing records in a table by changing the values of one or more columns for specified rows.
VALUES
The VALUES keyword is used to specify the data that will be inserted into a table in an SQL INSERT statement. It allows you to define one or more rows of data to be added to the specified columns of the table.
VIEW
The VIEW keyword is used to create a virtual table in a database that is based on the result of a SELECT query. It allows users to simplify complex queries, encapsulate logic, and present data in a specific format without storing the data physically.
WHERE
The WHERE keyword is used to filter records in a SQL query, allowing you to specify conditions that must be met for the records to be included in the result set.

Understanding the ALTER TABLE Command in SQL

The ALTER TABLE command is a powerful SQL statement used to modify an existing table structure in a database. It allows you to add, delete, or modify columns and constraints in a table, making it an essential tool for database management and design.

In simpler terms, ALTER TABLE lets you change the blueprint of your table without having to recreate it from scratch.

A Practical Example

Imagine you are managing a database for a library, and you have a table that stores information about books.

Books Table:

BookID Title Author YearPublished
1 The Great Gatsby F. Scott Fitzgerald 1925
2 1984 George Orwell 1949
3 To Kill a Mockingbird Harper Lee 1960

Now, suppose you want to add a new column to this table to store the genre of each book.

SQL Query Using ALTER TABLE

To add a new column called Genre, you would use the following SQL command:

SQL icon SQL 
ALTER TABLE Books
ADD Genre VARCHAR(50);

Result of the Query

After executing the above command, the Books table structure will now include the new Genre column:

BookID Title Author YearPublished Genre
1 The Great Gatsby F. Scott Fitzgerald 1925 NULL
2 1984 George Orwell 1949 NULL
3 To Kill a Mockingbird Harper Lee 1960 NULL

The Genre column has been added, and it currently contains NULL values for all existing records since no genre information has been provided yet.

Why Use ALTER TABLE?

The ALTER TABLE command is crucial for several reasons:

  • Flexibility: It allows you to adapt your database schema as requirements change without losing existing data.
  • Data Integrity: You can add constraints (like NOT NULL or UNIQUE) to ensure data quality and integrity.
  • Efficiency: Instead of creating a new table and migrating data, you can make changes directly to the existing table.

Key Takeaways:

  • ALTER TABLE: A command used to modify the structure of an existing table in a database.
  • Versatile Modifications: You can add, delete, or modify columns and constraints.
  • Essential for Database Management: It helps maintain the database's relevance and efficiency as business needs evolve.

Understanding the ALTER TABLE command is vital for anyone working with SQL databases. It empowers you to make necessary adjustments to your data structures, ensuring that your database remains functional and aligned with your evolving data needs.

Happy querying!

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