SELECT Statement SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id; SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; SELECT * FROM table1 LEFT JOIN table2 USING (id); SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id LEFT JOIN table3 ON table2.id = table3.id; Natural joins and joins with  USING , including outer join variants, are processed according to the SQL:2003 standard: •Redundant columns of a  NATURAL  join do not appear. Consider this set of statements: CREATE TABLE t1 (i INT, j INT); CREATE TABLE t2 (k INT, j INT); INSERT INTO t1 VALUES(1, 1); INSERT INTO t2 VALUES(1, 1); SELECT * FROM t1 NATURAL JOIN t2; SELECT * FROM t1 JOIN t2 USING (j); In the first  SELECT  statement, column  j  appears in both tables and thus becomes a join column, so, according to standard SQL, it should appear only once in the output, not twice. Similarly, in the second SELECT statement, column  j  is named in the  USING  clause and should appear only once in the output, not twice. Thus, the statements produce this output: Redundant column elimination and column ordering occurs according to standard SQL, producing this display order: •First, coalesced common columns of the two joined tables, in the order in which they occur in the first table •Second, columns unique to the first table, in order in which they occur in that table •Third, columns unique to the second table, in order in which they occur in that table +------+------+------+ | j | i | k | +------+------+------+ | 1 | 1 | 1 | +------+------+------+ +------+------+------+ | j | i | k | +------+------+------+ | 1 | 1 | 1 | +------+------+------+ COALESCE(x, y) = (CASE WHEN x IS NOT NULL THEN x ELSE y END) The single result column that replaces two common columns is defined using the coalesce operation. That is, for two  t1.a  and  t2.a  the resulting single join column  a  is defined as  a = COALESCE(t1.a, t2.a) , where: If the join operation is any other join, the result columns of the join consist of the concatenation of all columns of the joined tables. A consequence of the definition of coalesced columns is that, for outer joins, the coalesced column contains the value of the non- NULL  column if one of the two columns is always  NULL . If neither or both columns are  NULL , both common columns have the same value, so it doesn’t matter which one 2674