SQL Join,left Join,rigt Join

Left join: Connects all the records in the left table with the same records as the join fields in the right table.
Right join: Connects all the records in the right table with the same records as the join fields in the left table.
INNER JOIN: An inner join, also known as an equivalent connection, returns only rows in which the joined fields are equal in two tables.
Http://www.cnblogs.com/pcjim/articles/799302.html

The UNION operator is used to combine the result set of two or more SELECT statements.

Note that the SELECT statement inside the UNION must have the same number of columns. The column must also have a similar data type. Also, the order of the columns in each SELECT statement must be the same.

Http://www.w3school.com.cn/sql/sql_union.asp

SQL Join,left Join,rigt Join