1.
a. 并集UNION SELECT column1, column2 FROM table1 UNION SELECT column1, column2 FROM table2
b. 交集JOIN SELECT * FROM table1 AS a JOIN table2 b ON a.name=b.name
c. 差集NOT IN SELECT * FROM table1 WHERE name NOT IN(SELECT name FROM table2)
d. 笛卡尔积 SELECT * FROM table1 CROSS JOIN table2 与 SELECT * FROM table1,table2相同
2.
SQL中的UNION 与UNION ALL的区别是，前者会去除重复的条目，后者会仍旧保留。
a. UNION SQL Statement1 UNION SQL Statement2
b. UNION ALL SQL Statement1 UNION ALL SQL Statement2
3.
SQL中的各种JOIN SQL中的连接可以分为内连接，外连接，以及交叉连..