Update查询可以执行什么操作
更新查询使DBA或SQL使用程序员能够使用一个命令更新许多记录。
重要安全提示！在更改之前，请始终备份您要更改的内容！
本指南将：
- 在学生表中添加一个新字段
- 测试逻辑以使用学校指定的电子邮件地址更新该字段
- 更新新字段。
这是我们开始这个过程的学生表
SELECT * FROM student;
+-----------+------------------------+-----------+------------------+---------------------+---------------------+
| studentID | FullName | sat_score | programOfStudy | rcd_Created | rcd_Updated |
+-----------+------------------------+-----------+------------------+---------------------+---------------------+
| 1 | Monique Davis | 400 | Literature | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 2 | Teri Gutierrez | 800 | Programming | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 3 | Spencer Pautier | 1000 | Programming | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 4 | Louis Ramsey | 1200 | Programming | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 5 | Alvin Greene | 1200 | Programming | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 6 | Sophie Freeman | 1200 | Programming | 2017-08-16 15:34:50 | 2017-08-16 15:34:50 |
| 7 | Edgar Frank "Ted" Codd | 2400 | Computer Science | 2017-08-16 15:35:33 | 2017-08-16 15:35:33 |
| 8 | Donald D. Chamberlin | 2400 | Computer Science | 2017-08-16 15:35:33 | 2017-08-16 15:35:33 |
| 9 | Raymond F. Boyce | 2400 | Computer Science | 2017-08-16 15:35:33 | 2017-08-16 15:35:33 |
+-----------+------------------------+-----------+------------------+---------------------+---------------------+
9 rows in set (0.00 sec)
更改表格并添加新字段
ALTER TABLE `fcc_sql_guides_database`.`student`
ADD COLUMN `schoolEmailAdr` VARCHAR(125) NULL AFTER `programOfStudy`;
执行alter后的student表。
mysql> SELECT FullName, sat_score, programOfStudy, schoolEmailAdr FROM student;
+------------------------+-----------+------------------+----------------+
| FullName | sat_score | programOfStudy | schoolEmailAdr |
+------------------------+-----------+------------------+----------------+
| Monique Davis | 400 | Literature | NULL |
| Teri Gutierrez | 800 | Programming | NULL |
| Spencer Pautier | 1000 | Programming | NULL |
| Louis Ramsey | 1200 | Programming | NULL |
| Alvin Greene | 1200 | Programming | NULL |
| Sophie Freeman | 1200 | Programming | NULL |
| Edgar Frank "Ted" Codd | 2400 | Computer Science | NULL |
| Donald D. Chamberlin | 2400 | Computer Science | NULL |
| Raymond F. Boyce | 2400 | Computer Science | NULL |
+------------------------+-----------+------------------+----------------+
9 rows in set (0.00 sec)
测试逻辑（非常重要的一步！）
SELECT FullName, instr(FullName," ") AS firstSpacePosition,
concat(substring(FullName,1,instr(FullName," ")-1),"@someSchool.edu") AS schoolEmail
FROM student;
+------------------------+--------------------+------------------------+
| FullName | firstSpacePosition | schoolEmail |
+------------------------+--------------------+------------------------+
| Monique Davis | 8 | Monique@someSchool.edu |
| Teri Gutierrez | 5 | Teri@someSchool.edu |
| Spencer Pautier | 8 | Spencer@someSchool.edu |
| Louis Ramsey | 6 | Louis@someSchool.edu |
| Alvin Greene | 6 | Alvin@someSchool.edu |
| Sophie Freeman | 7 | Sophie@someSchool.edu |
| Edgar Frank "Ted" Codd | 6 | Edgar@someSchool.edu |
| Donald D. Chamberlin | 7 | Donald@someSchool.edu |
| Raymond F. Boyce | 8 | Raymond@someSchool.edu |
+------------------------+--------------------+------------------------+
9 rows in set (0.00 sec)
关于concat（）的注释：在MySQL中，此命令用于组合字符串，而在其他SQL版本中则不然（请查看手册）。在这种用法中，它的工作方式如下：FullName字段的子字符串（但不包括第一个空格）与“@ someSchool.edu”组合在一起。在现实世界中，这将变得更加复杂，您需要确保电子邮件地址是唯一的。
进行更新
我们假装这是我们想要的，并使用以下信息更新表：
UPDATE student SET schoolEmailAdr = concat(substring(FullName,1,instr(FullName," ")-1),"@someSchool.edu")
WHERE schoolEmailAdr is NULL;
成功！
mysql> SELECT FullName, sat_score, programOfStudy, schoolEmailAdr FROM student;
+------------------------+-----------+------------------+------------------------+
| FullName | sat_score | programOfStudy | schoolEmailAdr |
+------------------------+-----------+------------------+------------------------+
| Monique Davis | 400 | Literature | Monique@someSchool.edu |
| Teri Gutierrez | 800 | Programming | Teri@someSchool.edu |
| Spencer Pautier | 1000 | Programming | Spencer@someSchool.edu |
| Louis Ramsey | 1200 | Programming | Louis@someSchool.edu |
| Alvin Greene | 1200 | Programming | Alvin@someSchool.edu |
| Sophie Freeman | 1200 | Programming | Sophie@someSchool.edu |
| Edgar Frank "Ted" Codd | 2400 | Computer Science | Edgar@someSchool.edu |
| Donald D. Chamberlin | 2400 | Computer Science | Donald@someSchool.edu |
| Raymond F. Boyce | 2400 | Computer Science | Raymond@someSchool.edu |
+------------------------+-----------+------------------+------------------------+
9 rows in set (0.00 sec)
与所有这些SQL事物一样，它们比本入门指南中的内容更多。
我希望这至少足以让你开始。
请参阅您的数据库管理员手册，并自己尝试不同的选项。
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