Format:

          CREATE [PUBLIC | PRIVATE] ALIAS alias_name
             FOR {table_name | view_name};

or

          CREATE [PUBLIC | PRIVATE] SYNONYM synonym_name
             FOR {table_name | view_name};

An alias or synonym produces another name for a table or view.
The normal use of an alias or symonym is to avoid having to qualify another user’s table or view.
An alias or synonym simplifies the SQL statement and make it appear to be your own table.

A PRIVATE alias or synonym can only be accessed by the creator of the alias or synonym.
If unspecified, PRIVATE is the default.

A PUBLIC alias or synonym is accessible to all users of the database.
Normally only a Database Administrator or a privileged security user can create with PUBLIC.

Example:

          CREATE SYNONYM PILOTS FOR USER1.PILOT_TBL;

> Aliases and synonyms are not an SQL standard.

> DB2, Informix, and Oracle implement ALIAS and SYNONYM.

> MySQL does not implement ALIAS or SYNONYM.

DB2, Oracle, PostgreSQL Format:

          DROP INDEX index_name;

Access, MySQL Format:

          DROP INDEX index_name
            ON table_name;

SQL Server Format:

          DROP INDEX table_name.index_name;

Because an index is completely separate from its associated table, it can be deleted at any time without any effect on the base table other than performance issues. All SQL code will continue to function normally.

Most RDBMS systems have tools for measuring performance. If a table is small, or the data is infrequently referenced, you may choose to remove the index.

The SQL standard does not mention indices so vendors do their own implementation.

Format:

          CREATE {UNIQUE} INDEX index_name
              ON table_name (
                   column_name1 {ASC | DESC},
                   column_name2 {ASC | DESC},
                   ...);

A simple index contains only a single column.

A composite index contains multiple columns from a single table.
The index applies to all the columns in the index definition.
These columns do not have to be adjacent in the table.
Instead, they can be taken from any column in any order.
The order in the index definition is important and is defined left to right, going from major to minor sort order.

Your can create multiple indices on the same table.

ASC means ascending sort order, which is the default and does not need to be specified.
DESC means descending sort order and must be specified to reverse the sequence.
UNIQUE forces the combination of indexed columns to be unique.
> Access, MySQL, and SQL Server index names must be unique within a table.

> DB2, Oracle, and PostgreSQL index names must be unique within the database.