KodeKloud Notes

Structured Query Language (SQL) is the industry-standard language for interacting with relational databases. Pronounced “S-Q-L” or “sequel,” SQL is governed by ANSI/ISO, with each vendor—such as Microsoft’s T-SQL or Oracle’s PL/SQL—adding its own extensions. Mastering standard SQL lets you work across virtually any relational database.

Although SQL suggests only querying, it actually consists of three sublanguages:

Data Manipulation Language (DML): retrieve and modify data
Data Definition Language (DDL): create, alter, and drop database objects
Data Control Language (DCL): manage user permissions

The image is an informational graphic about Structured Query Language (SQL), explaining its ANSI/ISO standard, common names, vendor-specific features, and its three parts: Data Manipulation Language (DML), Data Definition Language (DDL), and Data Control Language (DCL).

Data Manipulation Language (DML)

DML is where you spend most of your SQL time. Each DML statement begins with one of these commands:

Command	Purpose	Example
SELECT	Retrieve rows from a table	`SELECT * FROM Customer;`
UPDATE	Modify existing rows	`UPDATE Customer SET ... WHERE ...;`
DELETE	Remove rows	`DELETE FROM Customer WHERE ...;`
INSERT	Add new rows	`INSERT INTO ... VALUES (...);`

Common clauses include:

FROM: specify source table(s)
JOIN: combine rows from multiple tables
WHERE: filter rows affected by the statement

SELECT

Retrieve data from a Customer table:

SELECT CustomerId,
       FirstName,
       LastName
FROM Customer
WHERE CustomerId = '0001';

This returns the row where CustomerId equals 0001.

UPDATE

Modify an existing row:

UPDATE Customer
SET LastName = 'Irvine'
WHERE CustomerId = '0001';

Only the LastName for customer 0001 is updated.

DELETE

Warning

Always include a WHERE clause in a DELETE statement. Omitting it will remove all rows from the table.

DELETE
FROM Customer
WHERE CustomerId = '0001';

This deletes only the matching customer.

INSERT

Add a new record:

INSERT INTO Customer (CustomerId, FirstName, LastName)
VALUES ('0003', 'Jason', 'van de Velde');

This creates a new row with the specified values.

Data Definition Language (DDL)

DDL statements manage database structures—tables, views, indexes, and more.

Command	Purpose	Example
CREATE	Add a new table, view, index, or stored procedure	`CREATE TABLE Orders (...);`
DROP	Remove an existing object	`DROP TABLE Orders;`
ALTER	Modify an existing object (e.g., add a column)	`ALTER TABLE Customer ADD Email VARCHAR;`

The image describes three additional statements in Data Definition Language: "Create" to add a table, "Drop" to remove a table, and "Alter" to add, rename, or change columns in a table.

Data Control Language (DCL)

DCL commands grant or revoke permissions to users and roles:

Command	Purpose	Example
GRANT	Give permissions (SELECT, INSERT, UPDATE, DELETE)	`GRANT SELECT ON Customer TO report_user;`
REVOKE	Remove previously granted permissions	`REVOKE DELETE ON Orders FROM temp_backup_role;`

The image explains two Data Control Language (DCL) statements: "Grant" to allow user access to a table, and "Revoke" to remove user access, both for DDL and DML operations.

References

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