Data Definition

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          CREATE, to create a new database object
          ALTER, to change an aspect of the structure of an existing database object
           DROP, to drop (remove) a database object

Data Manipulation and Transactions

         INSERT, to add rows to a table
         UPDATE, to change column values of existing rows
         DELETE, to remove rows from a table

COMMIT, to confirm all pending changes of the current transaction
ROLLBACK, to cancel all pending changes and restore the original situation

DELETE empties a table; DROP removes a table. TRUNCATE allows you to delete all the rows in a table in an efficient (but irrevocable) way.
UPDATE changes the contents of a table; ALTER changes its structure.
You can undo the consequences of data manipulation with ROLLBACK; data definition commands are irrevocable.