DDL

Data Definition Language (DDL)
statements are used to define the database structure or schema. Some examples:

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CREATE - to create objects in the database
ALTER - alters the structure of the database
DROP - delete objects from the database
TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
COMMENT - add comments to the data dictionary
RENAME - rename an object

DML

Data Manipulation Language (DML)
statements are used for managing data within schema objects. Some examples:

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SELECT - retrieve data from the a database
INSERT - insert data into a table
UPDATE - updates existing data within a table
DELETE - deletes all records from a table, the space for the records remain
MERGE - UPSERT operation (insert or update)
CALL - call a PL/SQL or Java subprogram
EXPLAIN PLAN - explain access path to data
LOCK TABLE - control concurrency

DCL

Data Control Language (DCL)
Some examples:

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GRANT - gives user's access privileges to database
REVOKE - withdraw access privileges given with the GRANT command

TCL

Transaction Control (TCL)
statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

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COMMIT - save work done
SAVEPOINT - identify a point in a transaction to which you can later roll back
ROLLBACK - restore database to original since the last COMMIT
SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use