Difference between DDL, DML, DCL and TCL

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

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:

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) statements. Some examples:

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.

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

Difference between Varchar, nVarchar, Char and nChar

VARCHAR and NVARCHAR
VARCHAR and NVARCHAR data types are both character data types that are variable-length. Below is the summary of the differences between these 2 data types:

You would use NVARCHAR data type for columns that store characters from more than one character set or when you will be using characters that require 2-byte characters, which are basically the Unicode characters such as the Japanese Kanji or Korean Hangul characters.

CHAR and VARCHAR
CHAR and VARCHAR data types are both non-Unicode character data types with a maximum length of 8,000 characters. The main difference between these 2 data types is that a CHAR data type is fixed-length while a VARCHAR is variable-length. If the number of characters entered in a CHAR data type column is less than the declared column length, spaces are appended to it to fill up the whole length.

Another difference is in the storage size wherein the storage size for CHAR is n bytes while for VARCHAR is the actual length in bytes of the data entered (and not n bytes).
You should use CHAR data type when the data values in a column are expected to be consistently close to the same size. On the other hand, you should use VARCHAR when the data values in a column are expected to vary considerably in size.

NCHAR and NVARCHAR
NCHAR and NVARCHAR data types are both Unicode character data types with a maximum length of 4,000 characters. The main difference between these 2 data types is that an NCHAR data type is fixed-length while an NVARCHAR is variable-length. If the number of characters entered in an NCHAR data type column is less than the specified column length, spaces are appended to it to fill up the whole length.

Another difference is in the storage size wherein the storage size for NCHAR is two times n bytes while for NVARCHAR is two times the number of characters entered (in bytes).
You should use NCHAR data type when the data values in a column are expected to be consistently close to the same size. On the other hand, you should use NVARCHAR when the data values in a column are expected to vary considerably in size.

CHAR and NCHAR
CHAR and NCHAR data types are both character data types that are fixed-length. Below is the summary of the differences between these 2 data types:

You would use NCHAR data type for columns that store characters from more than one character set or when you will be using characters that require 2-byte characters, which are basically the Unicode characters such as the Japanese Kanji or Korean Hangul characters.