一、需求

有兩個表格，test和test2如下所示：

mysql> select * from test;
+------+------+------------+
| data | id   | timestamp  |
+------+------+------------+
|  300 |    3 | 1536292800 |
|  200 |    2 | 1536292808 |
+------+------+------------+

mysql> select * from test2;
+------+------+
| data | id   |
+------+------+
 

|   10 |    3 |
|   20 |    2 |
+------+------+

1. 需求A

在test表格上直接操作，增加一列為context_time，資料型別為datetime，並賦值為對應的行的UNIX time的datetime，如下表所示（可以延伸為對每一行的某個欄位進行相同的運算或函式操作後賦值到新的欄位）：

+------+------+------------+---------------------+
| data | id   | timestamp  | context_time        |
+------+------+------------+---------------------+
 

|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |
+------+------+------------+---------------------+

2. 需求B

將表格test2中的data欄位查詢出來後根據id拼接到test表格後面，欄位名為new_data，如下所示：

+------+------+------------+---------------------+----------+
| data | id   | timestamp  | context_time        | new_data |
+------+------+------------+---------------------+----------+
 

|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |       10 |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |       20 |
+------+------+------------+---------------------+----------+

二、解決方案

1. 需求A的解決方案

• 先使用alter命令在原表格test上建立新的欄位context_time，資料型別為datetime，預設不為NULL

mysql> alter table test add context_time datetime not NULL;

mysql> select * from test;
+------+------+------------+---------------------+
| data | id   | timestamp  | context_time        |
+------+------+------------+---------------------+
|  300 |    3 | 1536292800 | 0000-00-00 00:00:00 |
|  200 |    2 | 1536292808 | 0000-00-00 00:00:00 |
+------+------+------------+---------------------+

• 然後使用update命令更新context_time的值：

mysql> update test set context_time=from_unixtime(timestamp);

mysql> select * from test;
+------+------+------------+---------------------+
| data | id   | timestamp  | context_time        |
+------+------+------------+---------------------+
|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |
+------+------+------------+---------------------+

2. 需求B的解決方案

• 先使用alter命令在原表格test上建立新的欄位new_data，資料型別為int，預設為NULL

mysql> alter table test add new_data int;

mysql> select * from test;
+------+------+------------+---------------------+----------+
| data | id   | timestamp  | context_time        | new_data |
+------+------+------------+---------------------+----------+
|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |     NULL |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |     NULL |
+------+------+------------+---------------------+----------+

• 然後使用update、inner join、select共同實現更新值：

mysql> update test inner join (select data, id from test2)tmp on test.id=tmp.id set test.new_data=tmp.data;

mysql> select * from test;
+------+------+------------+---------------------+----------+
| data | id   | timestamp  | context_time        | new_data |
+------+------+------------+---------------------+----------+
|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |       10 |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |       20 |
+------+------+------------+---------------------+----------+

三、結論

• alter命令用於修改表結構，update命令用於修改或更新表的資料（值）
• 不管怎樣，要增加一列新的資料，必須先使用alter命令修改原表的結構，增加新的欄位、資料型別和預設值
• alter命令還可以修改欄位名、調整欄位順序等
• 如果是需求A，僅僅對原表已有欄位進行相同操作後作為新欄位，那麼使用最簡單的update、set命令即可
• 如果是需求B，使用inner join就是類似表聯結的問題了，必須有主鍵（id）對應每一行資料，否則更新後的資料整一列都將是查詢的第一個資料的值，像這樣：

mysql> update test inner join (select data from test2)tmp set test.new_data=tmp.data;

mysql> select * from test;
+------+------+------------+---------------------+----------+
| data | id   | timestamp  | context_time        | new_data |
+------+------+------------+---------------------+----------+
|  300 |    3 | 1536292800 | 2018-09-07 12:00:00 |       10 |
|  200 |    2 | 1536292808 | 2018-09-07 12:00:08 |       10 |
+------+------+------------+---------------------+----------+

mysql> select data from test2;
+------+
| data |
+------+
|   10 |
|   20 |
+------+

• 另外，對於需求B，搜尋解決辦法的時候遇到有如下的寫法，但是mysql中是不可以這樣用的，其他SQL server我就沒嘗試了（我是以學習mysql為主=。=）

mysql> update test set test.new_data=(select data from test2);
ERROR 1242 (21000): Subquery returns more than 1 row

• 最後，一開始我是在折騰insert指令的，但是其實insert是插入行資料，和我想要的並不一樣，舉個栗子（會增加行然後預設填充為預設值，這裡是NULL）：

mysql> select * from test;
+------+------+
| data | id   |
+------+------+
|  100 |    1 |
|  200 |    2 |
+------+------+

mysql> alter table test add new_data int;

mysql> select * from test;
+------+------+----------+
| data | id   | new_data |
+------+------+----------+
|  100 |    1 |     NULL |
|  200 |    2 |     NULL |
+------+------+----------+

mysql> insert into test(new_data) select (data+1) as new_data from test;

mysql> select * from test;
+------+------+----------+
| data | id   | new_data |
+------+------+----------+
|  100 |    1 |     NULL |
|  200 |    2 |     NULL |
| NULL | NULL |      101 |
| NULL | NULL |      201 |
+------+------+----------+