我有 2 个表，其中包含以下列: 表1
+--------------+---------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+---------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| symbol | varchar(45) | NO | MUL | NULL | |
| v_dt | date | NO | | NULL | |
| e_dt | date | NO | | NULL | |
| col_s | decimal(10,4) | NO | | NULL | |
| col_o | decimal(10,4) | NO | | NULL | |
| col_b | decimal(10,4) | NO | | NULL | |
| col_a | decimal(10,4) | NO | | NULL | |
| col_l | decimal(10,4) | NO | | NULL | |
| col_v | bigint(20) | NO | | NULL | |
| col_t | enum('a','b') | NO | | NULL | |
+--------------+---------------+------+-----+---------+----------------+
和表 2:
+------------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+---------------+------+-----+---------+-------+
| t_date | date | NO | PRI | NULL | |
| e_date | date | NO | | NULL | |
| symbol | varchar(45) | NO | PRI | NULL | |
| col_i | decimal(10,6) | NO | | NULL | |
| col_b | decimal(10,6) | NO | | NULL | |
| col_d | decimal(10,6) | NO | | NULL | |
| col_g | decimal(10,6) | NO | | NULL | |
| col_v | decimal(10,6) | NO | | NULL | |
| col_t | decimal(10,6) | NO | | NULL | |
| col_r | decimal(10,6) | NO | | NULL | |
+------------+---------------+------+-----+---------+-------+
我想检索table1.col_b、table1.col_a和table2.col_d 所以，我写了以下内容
SELECT col_b, col_a, col_d FROM table1 LEFT JOIN table2 ON table1.symbol=table2.symbol;
但它没有返回应有的 40 行，而是继续运行以检索具有不相关数据的 4000 行，因此我猜测我错误地编写了 JOIN。 你可以吗？让我知道编写此代码以检索所需数据的正确方法是什么。谢谢! INNER JOIN 的结果(我更改了查询以包含日期，以便结果更有意义，我希望)
SELECT v_dt, e_dt, col_d FROM table1 INNER JOIN table2 ON table1.symbol=table2.symbol;
| 2011-09-26 | 2011-11-18 | 2.030130 |
| 2011-09-27 | 2011-11-18 | 2.030130 |
| 2011-09-28 | 2011-11-18 | 2.030130 |
| 2011-09-29 | 2011-11-18 | 2.030130 |
| 2011-09-30 | 2011-11-18 | 2.030130 |
| 2011-10-03 | 2011-11-18 | 2.030130 |
| 2011-10-04 | 2011-11-18 | 2.030130 |
| 2011-10-05 | 2011-11-18 | 2.030130 |
| 2011-10-06 | 2011-11-18 | 2.030130 |
| 2011-10-07 | 2011-11-18 | 2.030130 |
| 2011-10-10 | 2011-11-18 | 2.030130 |
| 2011-10-11 | 2011-11-18 | 2.030130 |
| 2011-10-12 | 2011-11-18 | 2.030130 |
| 2011-10-13 | 2011-11-18 | 2.030130 |
| 2011-10-14 | 2011-11-18 | 2.030130 |
| 2011-08-09 | 2011-11-18 | 1.628250 |
| 2011-08-10 | 2011-11-18 | 1.628250 |
| 2011-08-11 | 2011-11-18 | 1.628250 |
| 2011-08-12 | 2011-11-18 | 1.628250 |
| 2011-08-15 | 2011-11-18 | 1.628250 |
| 2011-08-16 | 2011-11-18 | 1.628250 |
| 2011-08-17 | 2011-11-18 | 1.628250 |
| 2011-08-18 | 2011-11-18 | 1.628250 |
| 2011-08-19 | 2011-11-18 | 1.628250 |
| 2011-08-22 | 2011-11-18 | 1.628250 |
| 2011-08-24 | 2011-11-18 | 1.628250 |
| 2011-08-25 | 2011-11-18 | 1.628250 |
| 2011-08-26 | 2011-11-18 | 1.628250 |
| 2011-08-29 | 2011-11-18 | 1.628250 |
| 2011-08-30 | 2011-11-18 | 1.628250 |
| 2011-08-31 | 2011-11-18 | 1.628250 |
| 2011-09-01 | 2011-11-18 | 1.628250 |
| 2011-09-02 | 2011-11-18 | 1.628250 |
| 2011-09-06 | 2011-11-18 | 1.628250 |
| 2011-09-07 | 2011-11-18 | 1.628250 |
| 2011-09-08 | 2011-11-18 | 1.628250 |
| 2011-09-09 | 2011-11-18 | 1.628250 |
| 2011-09-13 | 2011-11-18 | 1.628250 |
| 2011-09-14 | 2011-11-18 | 1.628250 |
| 2011-09-15 | 2011-11-18 | 1.628250 |
| 2011-09-16 | 2011-11-18 | 1.628250 |
| 2011-09-19 | 2011-11-18 | 1.628250 |
| 2011-09-20 | 2011-11-18 | 1.628250 |
| 2011-09-21 | 2011-11-18 | 1.628250 |
| 2011-09-22 | 2011-11-18 | 1.628250 |
| 2011-09-23 | 2011-11-18 | 1.628250 |
| 2011-09-26 | 2011-11-18 | 1.628250 |
| 2011-09-27 | 2011-11-18 | 1.628250 |
| 2011-09-28 | 2011-11-18 | 1.628250 |
| 2011-09-29 | 2011-11-18 | 1.628250 |
| 2011-09-30 | 2011-11-18 | 1.628250 |
| 2011-10-03 | 2011-11-18 | 1.628250 |
| 2011-10-04 | 2011-11-18 | 1.628250 |
| 2011-10-05 | 2011-11-18 | 1.628250 |
| 2011-10-06 | 2011-11-18 | 1.628250 |
| 2011-10-07 | 2011-11-18 | 1.628250 |
| 2011-10-10 | 2011-11-18 | 1.628250 |
| 2011-10-11 | 2011-11-18 | 1.628250 |
| 2011-10-12 | 2011-11-18 | 1.628250 |
| 2011-10-13 | 2011-11-18 | 1.628250 |
| 2011-10-14 | 2011-11-18 | 1.628250 |
| 2011-08-09 | 2011-11-18 | 1.254390 |
| 2011-08-10 | 2011-11-18 | 1.254390 |
| 2011-08-11 | 2011-11-18 | 1.254390 |
| 2011-08-12 | 2011-11-18 | 1.254390 |
| 2011-08-15 | 2011-11-18 | 1.254390 |
| 2011-08-16 | 2011-11-18 | 1.254390 |
| 2011-08-17 | 2011-11-18 | 1.254390 |
| 2011-08-18 | 2011-11-18 | 1.254390 |
| 2011-08-19 | 2011-11-18 | 1.254390 |
| 2011-08-22 | 2011-11-18 | 1.254390 |
| 2011-08-24 | 2011-11-18 | 1.254390 |
| 2011-08-25 | 2011-11-18 | 1.254390 |
| 2011-08-26 | 2011-11-18 | 1.254390 |
| 2011-08-29 | 2011-11-18 | 1.254390 |
| 2011-08-30 | 2011-11-18 | 1.254390 |
| 2011-08-31 | 2011-11-18 | 1.254390 |
| 2011-09-01 | 2011-11-18 | 1.254390 |
| 2011-09-02 | 2011-11-18 | 1.254390 |
| 2011-09-06 | 2011-11-18 | 1.254390 |
| 2011-09-07 | 2011-11-18 | 1.254390 |
| 2011-09-08 | 2011-11-18 | 1.254390 |
| 2011-09-09 | 2011-11-18 | 1.254390 |
| 2011-09-13 | 2011-11-18 | 1.254390 |
| 2011-09-14 | 2011-11-18 | 1.254390 |
| 2011-09-15 | 2011-11-18 | 1.254390 |
| 2011-09-16 | 2011-11-18 | 1.254390 |
| 2011-09-19 | 2011-11-18 | 1.254390 |
| 2011-09-20 | 2011-11-18 | 1.254390 |
| 2011-09-21 | 2011-11-18 | 1.254390 |
| 2011-09-22 | 2011-11-18 | 1.254390 |
| 2011-09-23 | 2011-11-18 | 1.254390 |
| 2011-09-26 | 2011-11-18 | 1.254390 |
| 2011-09-27 | 2011-11-18 | 1.254390 |
| 2011-09-28 | 2011-11-18 | 1.254390 |
| 2011-09-29 | 2011-11-18 | 1.254390 |
| 2011-09-30 | 2011-11-18 | 1.254390 |
| 2011-10-03 | 2011-11-18 | 1.254390 |
| 2011-10-04 | 2011-11-18 | 1.254390 |
| 2011-10-05 | 2011-11-18 | 1.254390 |
| 2011-10-06 | 2011-11-18 | 1.254390 |
| 2011-10-07 | 2011-11-18 | 1.254390 |
| 2011-10-10 | 2011-11-18 | 1.254390 |
| 2011-10-11 | 2011-11-18 | 1.254390 |
| 2011-10-12 | 2011-11-18 | 1.254390 |
| 2011-10-13 | 2011-11-18 | 1.254390 |
| 2011-10-14 | 2011-11-18 | 1.254390 |
| 2011-08-09 | 2011-11-18 | 1.019710 |
| 2011-08-10 | 2011-11-18 | 1.019710 |
| 2011-08-11 | 2011-11-18 | 1.019710 |
| 2011-08-12 | 2011-11-18 | 1.019710 |
| 2011-08-15 | 2011-11-18 | 1.019710 |
| 2011-08-16 | 2011-11-18 | 1.019710 |
| 2011-08-17 | 2011-11-18 | 1.019710 |
| 2011-08-18 | 2011-11-18 | 1.019710 |
| 2011-08-19 | 2011-11-18 | 1.019710 |
我所希望得到的是表 1 中的以下 2 列和表 2 中的第三列 col_d。因此我预计有 45 行。
+--------------+------------+
| 2011-08-09 | 2013-01-18 |
| 2011-08-10 | 2013-01-18 |
| 2011-08-11 | 2013-01-18 |
| 2011-08-12 | 2013-01-18 |
| 2011-08-15 | 2013-01-18 |
| 2011-08-16 | 2013-01-18 |
| 2011-08-17 | 2013-01-18 |
| 2011-08-18 | 2013-01-18 |
| 2011-08-19 | 2013-01-18 |
| 2011-08-22 | 2013-01-18 |
| 2011-08-24 | 2013-01-18 |
| 2011-08-25 | 2013-01-18 |
| 2011-08-26 | 2013-01-18 |
| 2011-08-29 | 2013-01-18 |
| 2011-08-30 | 2013-01-18 |
| 2011-08-31 | 2013-01-18 |
| 2011-09-01 | 2013-01-18 |
| 2011-09-02 | 2013-01-18 |
| 2011-09-06 | 2013-01-18 |
| 2011-09-07 | 2013-01-18 |
| 2011-09-08 | 2013-01-18 |
| 2011-09-09 | 2013-01-18 |
| 2011-09-13 | 2013-01-18 |
| 2011-09-14 | 2013-01-18 |
| 2011-09-15 | 2013-01-18 |
| 2011-09-16 | 2013-01-18 |
| 2011-09-20 | 2013-01-18 |
| 2011-09-21 | 2013-01-18 |
| 2011-09-22 | 2013-01-18 |
| 2011-09-23 | 2013-01-18 |
| 2011-09-26 | 2013-01-18 |
| 2011-09-27 | 2013-01-18 |
| 2011-09-28 | 2013-01-18 |
| 2011-09-29 | 2013-01-18 |
| 2011-09-30 | 2013-01-18 |
| 2011-10-03 | 2013-01-18 |
| 2011-10-04 | 2013-01-18 |
| 2011-10-05 | 2013-01-18 |
| 2011-10-06 | 2013-01-18 |
| 2011-10-07 | 2013-01-18 |
| 2011-10-10 | 2013-01-18 |
| 2011-10-11 | 2013-01-18 |
| 2011-10-12 | 2013-01-18 |
| 2011-10-13 | 2013-01-18 |
| 2011-10-14 | 2013-01-18 |
+--------------+------------+
45 rows in set (0.02 sec)
添加 GROUP BY v_dt, e_dt 使其减少到 45 行，并且 table1 的列是正确的。唯一的问题是它现在为 table2.col_d 显示相同的值(5.530000)，这不是它应该的值:-(
+--------------+------------+----------+
| 2011-08-09 | 2013-01-18 | 5.530000 |
| 2011-08-10 | 2013-01-18 | 5.530000 |
| 2011-08-11 | 2013-01-18 | 5.530000 |
| 2011-08-12 | 2013-01-18 | 5.530000 |
| 2011-08-15 | 2013-01-18 | 5.530000 |
| 2011-08-16 | 2013-01-18 | 5.530000 |
| 2011-08-17 | 2013-01-18 | 5.530000 |
| 2011-08-18 | 2013-01-18 | 5.530000 |
| 2011-08-19 | 2013-01-18 | 5.530000 |
| 2011-08-22 | 2013-01-18 | 5.530000 |
| 2011-08-24 | 2013-01-18 | 5.530000 |
| 2011-08-25 | 2013-01-18 | 5.530000 |
| 2011-08-26 | 2013-01-18 | 5.530000 |
| 2011-08-29 | 2013-01-18 | 5.530000 |
| 2011-08-30 | 2013-01-18 | 5.530000 |
| 2011-08-31 | 2013-01-18 | 5.530000 |
| 2011-09-01 | 2013-01-18 | 5.530000 |
| 2011-09-02 | 2013-01-18 | 5.530000 |
| 2011-09-06 | 2013-01-18 | 5.530000 |
| 2011-09-07 | 2013-01-18 | 5.530000 |
| 2011-09-08 | 2013-01-18 | 5.530000 |
| 2011-09-09 | 2013-01-18 | 5.530000 |
| 2011-09-13 | 2013-01-18 | 5.530000 |
| 2011-09-14 | 2013-01-18 | 5.530000 |
| 2011-09-15 | 2013-01-18 | 5.530000 |
| 2011-09-16 | 2013-01-18 | 5.530000 |
| 2011-09-20 | 2013-01-18 | 5.530000 |
| 2011-09-21 | 2013-01-18 | 5.530000 |
| 2011-09-22 | 2013-01-18 | 5.530000 |
| 2011-09-23 | 2013-01-18 | 5.530000 |
| 2011-09-26 | 2013-01-18 | 5.530000 |
| 2011-09-27 | 2013-01-18 | 5.530000 |
| 2011-09-28 | 2013-01-18 | 5.530000 |
| 2011-09-29 | 2013-01-18 | 5.530000 |
| 2011-09-30 | 2013-01-18 | 5.530000 |
| 2011-10-03 | 2013-01-18 | 5.530000 |
| 2011-10-04 | 2013-01-18 | 5.530000 |
| 2011-10-05 | 2013-01-18 | 5.530000 |
| 2011-10-06 | 2013-01-18 | 5.530000 |
| 2011-10-07 | 2013-01-18 | 5.530000 |
| 2011-10-10 | 2013-01-18 | 5.530000 |
| 2011-10-11 | 2013-01-18 | 5.530000 |
| 2011-10-12 | 2013-01-18 | 5.530000 |
| 2011-10-13 | 2013-01-18 | 5.530000 |
| 2011-10-14 | 2013-01-18 | 5.530000 |
+--------------+------------+----------+
这是更新后的查询。
从 table1.symbol=table2.symbol group by v_dt、e_dt 上的 table1 内连接 table2 中选择 table1.v_dt、table1.e_dt、table2.col_b；
为了进一步缩小结果，我还跑了
从 table1.symbol='P00055000' group by v_dt、e_dt 上的 table1 内连接 table2 中选择 table1.v_dt、table1.e_dt、table2.col_b；
但我仍然得到相同的结果，然后
select symbol, count(*) from table2 where symbol='P00055000' group by symbol;
+--------------------+----------+
| P00055000 | 40 |
+--------------------+----------+
1 row in set (0.02 sec)
最佳答案
如果您有两个表 A 和 B:
- A LEFT JOIN B = 选择所有行 A 以及 B 中符合联接条件的所有行，如果 B 不符合联接条件，则选择 NULL
- A RIGHT JOIN B = 选择所有行 B 以及 A 中符合联接条件的任何行，如果 A 不符合联接条件，则选择 NULL
- A INNER JOIN B = 仅选择 A 和 B 中符合连接条件的行
对于连接表的左/右连接中选择的任何列，如果该行与连接条件不匹配，则将出现 NULL 值。
例如，假设我们有两张表
Table A (id, name, description)
Table B (id, a_id, group_id, date)
当我们运行下面的LEFT JOIN时，如果在B表中找不到记录，我们预计查询结果将为B.group_id和B.date返回NULL。
SELECT A.id, A.name, A.description, B.group_id, B.date FROM A
LEFT JOIN B ON B.a_id=A.id
结果
id | name | description | group_id | date
1 | Test | Test | 2 | 2011-3-4
2 | Test | Test | NULL | NULL
可以看到，第一行成功地在 A 和 B 中找到了符合连接条件的记录。另一方面，第 2 行在 B 中找不到匹配的记录，因此添加了 NULL 值。
让我们看一下RIGHT JOIN。这实际上是 LEFT JOIN 的逆操作。我们期望 B 中的所有记录都包含其数据，但如果 A 中的记录不匹配，它将包含 NULL。
SELECT A.id, A.name, A.description, B.group_id, B.date FROM A
RIGHT JOIN B ON B.a_id=A.id
结果
id | name | description | group_id | date
1 | Test | Test | 2 | 2011-3-4
NULL | NULL | NULL | 3 | 2011-5-6
如您所见，第一行成功地在 A 和 B 中找到了符合连接条件的记录(与 LEFT JOIN 相同)。另一方面，第 2 行在 A 中找不到匹配的记录，因此添加了 NULL 值。
最后让我们看看INNER JOIN。 INNER JOIN 通常是最有用的联接，因为它仅返回在 A 和 B 中匹配的记录，这通常是您要查找的内容
SELECT A.id, A.name, A.description, B.group_id, B.date FROM A
INNER JOIN B ON B.a_id=A.id
结果
id | name | description | group_id | date
1 | Test | Test | 2 | 2011-3-4
现在我们只返回 A 和 B 中都匹配的记录，而忽略其余的记录。希望这能为您解决问题。
关于mysql JOIN - 左连接、右连接还是外连接？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7784887/