如何从MySQL数据库表中检索数据
在《用MySQL创建数据库和数据库表》文章中，我们如何创建一个数据库和数据库表，并知道如何向数据库表中添加记录。那么我们如何从数据库表中检索数据呢？ 1、从数据库表中检索信息 实际上，前面我们已经用到了SELECT语句，它用来从数据库表中检索信息。 sele
在《用MySQL创建数据库和数据库表》文章中，我们如何创建一个数据库和数据库表，并知道如何向数据库表中添加记录。 那么我们如何从数据库表中检索数据呢？
1、从数据库表中检索信息
实际上，前面我们已经用到了SELECT语句，它用来从数据库表中检索信息。
select语句格式一般为：
SELECT 检索关键词 FROM 被检索的表 WHERE 检索条件(可选)
以前所使用的“ * ”表示选择所有的列。下面继续使用我们在上篇文章中创建的表mytable。
2、查询所有数据：
<ccid_code>mysql> select * from mytable;
+----------+------+------------+----------+
| name | sex | birth | birthaddr |
+----------+------+------------+--------+
| abccs |f | 1977-07-07 | china |
| mary |f | 1978-12-12 | usa |
| tom |m | 1970-09-02 | usa |
+----------+------+------------+----------+
3 row in set (0.00 sec)</ccid_code>
3、修正错误记录：
假如tom的出生日期有错误，应该是1973－09－02，则可以用update语句来修正： mysql> update mytable set birth = "1973-09-02" where name = "tom"; 再用2中的语句看看是否已更正过来。
4、选择特定行
上面修改了tom的出生日期，我们可以选择tom这一行来看看是否已经有了变化：
<ccid_code>mysql> select * from mytable where name = "tom"; +--------+------+------------+------------+ | name |sex | birth | birthaddr | +--------+------+------------+------------+ | tom |m | 1973-09-02 | usa | +--------+------+------------+------------+ 1 row in set (0.06 sec)</ccid_code> Copy after login |
上面WHERE的参数指定了检索条件。我们还可以用组合条件来进行查询：
<ccid_code>mysql> SELECT * FROM mytable WHERE sex = "f" AND birthaddr = "china"; +--------+------+------------+------------+ | name |sex | birth | birthaddr | +--------+------+------------+------------+ | abccs |f | 1977-07-07 | china | +--------+------+------------+------------+ 1 row in set (0.06 sec)</ccid_code> Copy after login |
5、 选择特定列
假如你想查看表中的所有人的姓名，则可以这样操作：
<ccid_code>mysql> SELECT name FROM mytable; +----------+ | name | +----------+ | abccs | | mary | | tom | +----------+ 3 row in set (0.00 sec)</ccid_code> Copy after login |
如果想列出姓名和性别两列，则可以用逗号将关键词name和birth分开： myaql> select name,birth from mytable;
6、对行进行排序
我们可以对表中的记录按生日大小进行排序：
<ccid_code>mysql> SELECT name, birth FROM mytable ORDER BY birth; +----------+------------+ | name | birth | +----------+------------+ | tom | 1973-09-02 | | abccs | 1977-07-07 | | mary | 1978-12-12 | +----------+------------+ 3 row in set (0.00 sec)</ccid_code> Copy after login |
我们可以用DESC来进行逆序排序：
<ccid_code>mysql> SELECT name, birth FROM mytable ORDER BY birth DESC; +----------+------------+ | name | birth | +----------+------------+ | mary | 1978-12-12 | | abccs | 1977-07-07 | | tom | 1973-09-02 | +----------+------------+ 3 row in set (0.00 sec)</ccid_code> Copy after login |
7、 行计数
数据库经常要统计一些数据，如表中员工的数目，我们就要用到行计数函数COUNT()。COUNT()函数用于对非NULL结果的记录进行计数：
<ccid_code>mysql> SELECT COUNT(*) FROM mytable; +----------+ | COUNT(*) | +----------+ | 3 | +----------+ 1 row in set (0.06 sec) 员工中男女数量： mysql> SELECT sex, COUNT(*) FROM mytable GROUP BY sex; +------+----------+ | sex | COUNT(*) | +------+----------+ | f | 2 | | m | 1 | +------+----------+ 2 row in set (0.00 sec)</ccid_code> Copy after login |
注意我们使用了GROUP BY对SEX进行了分组。
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