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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).

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Data Manipulation Language (DML)

DML is where you spend most of your SQL time. Each DML statement begins with one of these commands:
CommandPurposeExample
SELECTRetrieve rows from a tableSELECT * FROM Customer;
UPDATEModify existing rowsUPDATE Customer SET ... WHERE ...;
DELETERemove rowsDELETE FROM Customer WHERE ...;
INSERTAdd new rowsINSERT INTO ... VALUES (...);
Common clauses include:
  • FROM: specify source table(s)
  • JOIN: combine rows from multiple tables
  • WHERE: filter rows affected by the statement

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SELECT

Retrieve data from a Customer table: 
SELECT CustomerId,
       FirstName,
       LastName
FROM Customer
WHERE CustomerId = '0001';
This returns the row where CustomerId equals 0001.

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UPDATE

Modify an existing row: 
UPDATE Customer
SET LastName = 'Irvine'
WHERE CustomerId = '0001';
Only the LastName for customer 0001 is updated.

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DELETE

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.

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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.

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Data Definition Language (DDL)

DDL statements manage database structures—tables, views, indexes, and more.
CommandPurposeExample
CREATEAdd a new table, view, index, or stored procedureCREATE TABLE Orders (...);
DROPRemove an existing objectDROP TABLE Orders;
ALTERModify 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.

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Data Control Language (DCL)

DCL commands grant or revoke permissions to users and roles:
CommandPurposeExample
GRANTGive permissions (SELECT, INSERT, UPDATE, DELETE)GRANT SELECT ON Customer TO report_user;
REVOKERemove previously granted permissionsREVOKE 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.

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References

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