mysql查詢數據，根據條件更新到另一張表

原本的數據庫有3張表

• travel_way ：旅遊線路表，存放線路的具體信息
• traveltag ：線路標籤表，存放線路目的地等信息
• tagrelation：標籤對應表，存放線路和目的地的對應關係

因爲業務邏輯的改變，現在要把它們合併爲一張表，把traveltag中的目的地信息插入到travel_way中。

首先獲取到所有線路對應的目的地，以線路ID分組，合併目的地到一行，以逗號分隔。

SELECT travel_way.id,GROUP_CONCAT(traveltag.content) FROM travel_way LEFT JOIN tagrelation on travel_way.id = tagrelation.travel_id LEFT JOIN traveltag ON tagrelation.tag_id = traveltag.id GROUP BY travel_way.id

先把查到的數據存放到了一個新建的表mid裏

INSERT into mid (travelway_id,destination) SELECT travel_way.id,GROUP_CONCAT(traveltag.content) FROM travel_way LEFT JOIN tagrelation on travel_way.id = tagrelation.travel_id LEFT JOIN traveltag ON tagrelation.tag_id = traveltag.id GROUP BY travel_way.id

然後將mid表的數據更新到travel_way裏，因爲是更新，所以不能用insert into select from 語句了

update travel_way,mid set travel_way.destination = mid.destination where travel_way.id = mid.travelway_id

成功將目的地以逗號分隔的字符串形式導入travel_way表中

說一下用到的幾個方法，group_concat
group_concat( [DISTINCT] 要連接的字段 [Order BY 排序字段 ASC/DESC] [Separator ‘分隔符’] ),該函數能夠將相同的行組合起來

select * from goods;  

+------+------+
| id| price|
+------+------+
|1 | 10|
|1 | 20|
|1 | 20|
|2 | 20|
|3 | 200 |
|3 | 500 |
+------+------+
6 rows in set (0.00 sec)

以id分組，把price字段的值在同一行打印出來，逗號分隔(默認）

select id, group_concat(price) from goods group by id; 
+------+--------------------+
| id| group_concat(price) |
+------+--------------------+
|1 | 10,20,20|
|2 | 20 |
|3 | 200,500|
+------+--------------------+
3 rows in set (0.00 sec)

以id分組，把price字段去重打印在一行，逗號分隔

select id,group_concat(distinct price) from goods group by id;  
+------+-----------------------------+
| id| group_concat(distinct price) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20 |
|3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)

以id分組，把price字段的值打印在一行，逗號分隔，按照price倒序排列

select id,group_concat(price order by price desc) from goods group by id;  
+------+---------------------------------------+
| id| group_concat(price order by price desc) |
+------+---------------------------------------+
|1 | 20,20,10 |
|2 | 20|
|3 | 500,200|
+------+---------------------------------------+
3 rows in set (0.00 sec)

insert into select from 將查詢到的記錄插入到某個表中，
INSERT INTO db1_name(field1,field2) SELECT field1,field2 FROM db2_name
要求目標db2必須存在，下面測試一下，有兩個表，結構如下

select * from insert_one;
+----+--------+-----+-----+
| id | name   | age | sex |
+----+--------+-----+-----+
|  1 | 田小斯 | 25  |     |
|  2 | 劉大牛 | 26  |     |
|  3 | 鄭大錘 | 28  |     |
|  4 | 胡二狗 | 30  |     |
+----+--------+-----+-----+
4 rows in set

select * from insert_sex;
+----+-----+
| id | sex |
+----+-----+
|  1 | 1   |
|  2 | 2   |
|  3 | 1   |
|  4 | 2   |
+----+-----+
4 rows in set

從表2中查找性別數據，插入到表1中

into insert_one(sex) select sex from insert_sex;
Query OK, 4 rows affected
select * from insert_one;
+----+--------+-----+-----+
| id | name   | age | sex |
+----+--------+-----+-----+
|  1 | 田小斯 | 25  |     |
|  2 | 劉大牛 | 26  |     |
|  3 | 鄭大錘 | 28  |     |
|  4 | 胡二狗 | 30  |     |
|  5 |        |     | 1   |
|  6 |        |     | 2   |
|  7 |        |     | 1   |
|  8 |        |     | 2   |
+----+--------+-----+-----+
8 rows in set

結果很尷尬，我是想要更新這張表的sex字段，而不是插入新的數據，那麼這個命令只適用於要把數據導入空表中，所以在上面的實際需要中，我建立了新表mid，利用update來中轉並更新數據

UPDATE tb1,tb2 SET tb1.address=tb2.address WHERE tb1.name=tb2.name
根據條件匹配，把表1的數據替換爲（更新爲）表2的數據，表1和表2必須有關聯纔可以

update insert_one,insert_sex set insert_one.sex = insert_sex.sex where insert_one.id = insert_sex.id;
Query OK, 4 rows affected
select * from insert_one;
+----+--------+-----+-----+
| id | name   | age | sex |
+----+--------+-----+-----+
|  1 | 田小斯 | 25  | 1   |
|  2 | 劉大牛 | 26  | 2   |
|  3 | 鄭大錘 | 28  | 1   |
|  4 | 胡二狗 | 30  | 2   |
|  5 |        |     | 1   |
|  6 |        |     | 2   |
|  7 |        |     | 1   |
|  8 |        |     | 2   |
+----+--------+-----+-----+
8 rows in set

成功將數據更新到insert_one表的sex字段中。