A UNION operator combines the results of two or more selects, giving all the rows returned by each select as one single result table.

                                                        UNION syntax:

                                                 SELECT statement_1
                                                   UNION [ALL | DISTINCT]
                                                 SELECT statement_2
                                                   UNION [ALL | DISTINCT]
                                                 SELECT statement_3 …

UNION …….. establishes which result sets will be combined into one result set.
                    Duplicate rows and nulls are excluded by default.

ALL …………. means that duplicate rows are not to be excluded.

DISTINCT … means duplicate rows and nulls are to be excluded.
                    If neither ALL or DISTINCT are specified, then DISTINCT is the default.

> The multiple selects must have the same number of columns (column names, functions, or arithmetic expressions).

> The order of columns must match in data type or be convertible to the same data type.

> If the name in a column is the same in all selects, then this name is used in the result. If this is not true, then most RDBMS systems take the name from the first select. If you want to create a different column name, use an AS clause in the first select for a column alias.

> ORDER BY can only be used in the last select, but it is applied to the entire result table.

> GROUP BY and HAVING can be used in each select statement, but do not apply to the final result table.

> Access, DB2, Oracle, PostgreSQL, and SQL Server support UNION and UNION ALL.

> MySQL does not support UNION or UNION ALL.

A FULL OUTER JOIN includes all rows from the left table and all rows from the right table. Any table rows without matching table rows from the other table will have nulls returned for the missing selected values.

                                                  FULL OUTER JOIN syntax:

                                      SELECT t1.column1 AS ‘title1′, 
                                                   t2.column2 AS ‘title2′, …
                                         FROM left_table                          [AS] ‘t1′
                                          FULL [OUTER] JOIN right_table [AS] ‘t2′
                                             ON left_t1.column1 oper right_t2.column2[, ...]
                                     [GROUP BY t1.column1]
                                     [HAVING      t1.column1]
                                     [ORDER BY t1.column1 [ASC|DESC]]

If the tables have common column names, those column names must be qualified with the table name or alias as follows: [left_table.]column oper [right_table.]column.

> While FULL OUTER JOINs can be done occasionally without the key word OUTER, the word OUTER should normally be included in the OUTER JOIN statement for clarity of understanding.

> Access and MySQL do not support FULL OUTER JOIN, but it can be done with a UNION ALL.

> Oracle 9i and later supports the FULL OUTER JOIN, but it also has the non-standard (+) outer join operator.

> SQL Server supports FULL OUTER JOIN, but it also has the non-standard outer join * in the WHERE syntax.

A RIGHT OUTER JOIN includes all rows from the right table and all matching rows from the left table. Any right table rows without matching left table rows will have nulls returned for the missing selected values. The order of tables in a RIGHT OUTER JOIN are critical and can not be rearranged.

                                              RIGHT OUTER JOIN syntax:

                                    SELECT t1.column1 AS ‘title1′,
                                                 t2.column2 AS ‘title2′, …
                                       FROM left_table [AS] ‘t1′
                                      RIGHT [OUTER] JOIN right_table [AS] ‘t2′
                                           ON left_t1.column1 oper right_t2.column2[, ...]
                                   [GROUP BY t1.column1] 
                                   [HAVING      t1.column1] 
                                   [ORDER BY t1.column1 [ASC|DESC]]

If the tables have common column names, those column names must be qualified with the table name or alias as follows: [left_table.]column oper [right_table.]column.

> While RIGHT OUTER JOINs can be done occasionally without the key word OUTER, the word OUTER should normally be included in the OUTER JOIN statement for clarity of understanding.

> Oracle 9i and later supports the OUTER JOIN, but it also has the non-standard (+) outer join operator.

> SQL Server supports OUTER JOIN, but it also has the non-standard outer join * in the WHERE syntax.