笛卡尔积的理解
select id,department_name from employees,departments;#错的 select id,department_id,department_name from employees CROSS JOIN departments;#错的
每个员工和每个部门匹配了一遍（查出的条目数=id数*department数）
错误原因：缺少连接条件
编写连接条件: 表1.列 = 表二.列（若多个表连接，至少要用n-1个连接条件）
select id,employees.name,department_name from employees,departments WHERE employees.name = departments.name;
注：如果要显示的列在要查询的表中名字一样，则要表明，是出自哪个表， eg: employees.name
建议在多表查询时，标明显示的是哪个表的信息 （优化）
优化：可以在FROM后使用表的别名，但是一旦使用别名，后续就一定要都用别名
等值连接：上述的带有=的
非等值连接：没有=的
select t1.id,t1.name,t2.grade from employees t1,departments t2 WHERE ti.salary BETWEEN t2.lowest_salary AND t2.highest_salary ;#非等值
非自链接：表1和表2连接
自链接：表1和自己连接
#显示员工（t1）和其管理者（t2）的基本信息 select t1.id,t1.name,t2.id，t2.name from employees t1,employees t2#一个表看作两个表 WHERE t1.manage_id = t2.id ;#自连接
内连接：合并含有同一列的表，结果不包括一个表与另一个表不匹配打的行
外连接：合并含有同一列的表，结果除了内连接的结果还查询不匹配的行
外连接的分类：左外连接（左表多，补右边），右外连接（右表多，补左边），满外连接
内连接：见上
外连接：左表有数据不匹配，在右表加（+）；反之，在左表加（+），但是MySQL不支持
WHERE t1.department_id = t2.department_id(+)#左连接
select t1.id,t1.name,t2.department_name,t3.environment from employees t1 JOIN departments t2 ON t1.department_id = t2.department_id JOIN locations t3#加入第二个人表 ON t2.department_location = t3.department_location;
使用OUTER JOIN...ON...
左外连接：LEFT OUTER JOIN
右外连接：RIGHT OUTER JOIN
满外连接：FULL OUTER JOIN（MySQL不支持）
select t1.name,t2.department_name#左外连接 from employees t1 LEFT OUTER（可省略） JOIN departments t2 ON t1.department_id = t2.department_id；
合并查询结果
SELECT colum... FROM table1 UNION (ALL) SELECT colum... FROM table2
UNION操作符
两个查询结果的并集，去重（效率低）
UNION ALL操作符（推荐）
两个查询结果的并集，不去重（效率高）
7种SQL JOINS的实现
中图（内连接）：
select t1.name,t2.department_name from employees t1 JOIN departments t2 ON t1.department_id = t2.department_id；
左上图(左外连接)：
select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id；
右上图(右外连接)：
select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id；
左中图：
select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL;
右中图：
select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL;
左下图(满外连接)：
#方式一：左上图 UNION ALL 右中图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL; #方式二：左中图 UNION ALL 右上图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id；
右下图：
#左中图 UNION ALL 右中图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL;
使用关键字：NATURAL JOIN（不灵活），自动查询表中所有相同字段，然后进行等值连接
使用关键字：USING（同名字段），将表中相同名字的字段自动等值连接
select t1.name,t2.department_name from employees t1 JOIN departments t2 ON t1.department_id = t2.department_id; 等价于 select t1.name,t2.department_name from employees t1 JOIN departments t2 USING(department_id);
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