数据库中存储树状关系的数据
表1：字段A1,字段A2,字段A3.
字段A1是主键
表2：字段B1，字段B2，字段A1
字段B1是主键，字段A1是其外键。
1个字段A1的值可以匹配多条表2的记录.
表3：字段C1，字段C2，字段B1
字段C1是主键，字段B1是其外键.
1个字段B1的值可以匹配多条表3的记录.
-- 创建第一个表
CREATE TABLE Table1 (A1 INTEGER PRIMARY KEY,A2 TEXT,A3 TEXT
);-- 创建第二个表
CREATE TABLE Table2 (B1 INTEGER PRIMARY KEY,B2 TEXT,A1 INTEGER,FOREIGN KEY (A1) REFERENCES Table1(A1)
);CREATE TABLE Table3 (C1 INT PRIMARY KEY,C2 VARCHAR(255), -- 假设C2是字符串类型，长度为255字符B1 INT,FOREIGN KEY (B1) REFERENCES Table2(B1) -- 假设Table2中B1是主键或者有唯一约束
);
-- 插入 Table1 的数据
INSERT INTO Table1 (A1, A2, A3) VALUES (6, 'ValueA2_6', 'ValueA3_6');
INSERT INTO Table1 (A1, A2, A3) VALUES (7, 'ValueA2_7', 'ValueA3_7');
INSERT INTO Table1 (A1, A2, A3) VALUES (8, 'ValueA2_8', 'ValueA3_8');-- 插入 Table2 的数据
-- 假设 A1 的值 6, 7, 8 在 Table1 中已经存在
INSERT INTO Table2 (B1, B2, A1) VALUES (6, 'ValueB2_6', 6);
INSERT INTO Table2 (B1, B2, A1) VALUES (7, 'ValueB2_7', 6);
INSERT INTO Table2 (B1, B2, A1) VALUES (8, 'ValueB2_8', 6);
INSERT INTO Table2 (B1, B2, A1) VALUES (9, 'ValueB2_9', 7);
INSERT INTO Table2 (B1, B2, A1) VALUES (10, 'ValueB2_10', 7);
INSERT INTO Table2 (B1, B2, A1) VALUES (11, 'ValueB2_11', 7);
INSERT INTO Table2 (B1, B2, A1) VALUES (12, 'ValueB2_12', 8);
INSERT INTO Table2 (B1, B2, A1) VALUES (13, 'ValueB2_13', 8);
INSERT INTO Table2 (B1, B2, A1) VALUES (14, 'ValueB2_14', 8);
-- 插入 Table3 的数据
INSERT INTO Table3 (C1, C2, B1) VALUES (1, 'Record1', 6);
INSERT INTO Table3 (C1, C2, B1) VALUES (2, 'Record2', 7);
INSERT INTO Table3 (C1, C2, B1) VALUES (3, 'Record3', 8);
INSERT INTO Table3 (C1, C2, B1) VALUES (4, 'Record4', 9);
INSERT INTO Table3 (C1, C2, B1) VALUES (5, 'Record5', 10);
INSERT INTO Table3 (C1, C2, B1) VALUES (6, 'Record6', 11);
INSERT INTO Table3 (C1, C2, B1) VALUES (7, 'Record7', 12);
INSERT INTO Table3 (C1, C2, B1) VALUES (8, 'Record8', 13);
INSERT INTO Table3 (C1, C2, B1) VALUES (9, 'Record9', 14);
INSERT INTO Table3 (C1, C2, B1) VALUES (10, 'Record10', 6);
INSERT INTO Table3 (C1, C2, B1) VALUES (11, 'Record11', 7);
INSERT INTO Table3 (C1, C2, B1) VALUES (12, 'Record12', 8);
INSERT INTO Table3 (C1, C2, B1) VALUES (13, 'Record13', 9);
INSERT INTO Table3 (C1, C2, B1) VALUES (14, 'Record14', 10);
INSERT INTO Table3 (C1, C2, B1) VALUES (15, 'Record15', 11);
INSERT INTO Table3 (C1, C2, B1) VALUES (16, 'Record16', 12);
INSERT INTO Table3 (C1, C2, B1) VALUES (17, 'Record17', 13);
INSERT INTO Table3 (C1, C2, B1) VALUES (18, 'Record18', 14);
INSERT INTO Table3 (C1, C2, B1) VALUES (19, 'Record19', 6);
INSERT INTO Table3 (C1, C2, B1) VALUES (20, 'Record20', 7);
INSERT INTO Table3 (C1, C2, B1) VALUES (21, 'Record21', 8);
INSERT INTO Table3 (C1, C2, B1) VALUES (22, 'Record22', 9);
INSERT INTO Table3 (C1, C2, B1) VALUES (23, 'Record23', 10);
INSERT INTO Table3 (C1, C2, B1) VALUES (24, 'Record24', 11);
INSERT INTO Table3 (C1, C2, B1) VALUES (25, 'Record25', 12);
INSERT INTO Table3 (C1, C2, B1) VALUES (26, 'Record26', 13);
INSERT INTO Table3 (C1, C2, B1) VALUES (27, 'Record27', 14);
1.查询table2中A1 = '6'的记录
select * from table2 where A1 = '6';
2.查询table3中B1 = '6'的记录
select * from table3 where B1 = '6';
3.如果要查询table3中对应的A1=‘6’的记录呢？
select * from table3 where B1 in (select B1 from table2 where A1 = '6');
9条记录
4.思考：
select table3.*,table2.A1 from table3,table2 where table2.A1 = '6'
81条记录
这样写为什么不对？
因为缺少了条件table2.B1 = table3.B1
5.
select table3.*,table2.A1 from table3,table2 where table2.A1 = '6' and table2.B1 = table3.B1
这样就对了
同义语句：
select table3.*,table2.A1 from table3 inner join table2 on table2.A1 = '6' and table2.B1 = table3.B1
------
6.观察：
select table3.*,table2.* from table3,table2
Table2和Table3联合起来：
243条记录
table2有9条，table3有27条。
所以，实际上是9*27条
(笛卡尔积是指两个集合中所有可能的组合)（也就是上面这种情况）
------