我想从 id = 4 的数据库表中返回字段 name。MYSQL 更容易处理哪个查询。
SELECT name, id FROM table WHERE id = 4
或
SELECT name FROM table WHERE id = 4
我想我想问是否将WHERE 语句中的字段也放入SELECT 是否是一种好习惯，即使我不会在php 中返回它。
【问题讨论】：
SELECT name, id FROM table WHERE id = 4
在mysql的结果中也会有id，它对数据的影响很小，因为它只是一个数字，但是如果你在选择其他东西的地方这样做，比如SELECT name, id FROM table WHERE name = "ron"肯定会不好的做法是从数据库服务器传输更多数据，因为您已经知道名称“ron”，并且由于它不是唯一的，它可能有不止一行。
mysql> explain select id,name from admin_users where id = 1;
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
mysql> explain select name from admin_users where id = 1;
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
相比
mysql> explain select id,name from admin_users where name = 'ron';
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
123 row in set (0.11 sec)
mysql> explain select name from admin_users where name = 'ron';
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
123 row in set (0.05 sec)
【讨论】：
如果您不需要返回的 id，请不要包含它。如果需要，请包含它。
【讨论】：
不，这不是好习惯。 SELECT 正是您需要的那些字段。
【讨论】：
我想我是在问将字段从 SELECT 中的 WHERE 语句也是如此，即使我不会在 php 中返回它。
不。当数据在客户端服务器之间传输时，这将增加网络开销。如果您不打算使用数据，请不要选择它。
【讨论】：
除非您在结果中需要“id”列，否则没有理由要求它。 MySQL 将同样对待查询。请求它可能会减慢速度，因为 MySQL 必须向您返回额外的数据，并且您的程序将需要解析它。怀疑它对“id”列很重要，但如果它是几个巨大的文本字段，它可能会。
这是 MySQL 对我的一个简单表上的两个查询的看法（id 字段上有一个索引）。
mysql> explain select id,name from admin_users where id = 1;
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
mysql> explain select name from admin_users where id = 1;
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | admin_users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
【讨论】：
admin_users 表的记录数，但是如果您进行详细的分析，您肯定会看到以微秒为单位的差异。
我怀疑第二种选择，在大型数据库（许多记录 > 1M）上更快，因为从数据库中提取数据所需的时间更少。
这是对一个小表（1187 条记录）进行的分析，id 是主键：
mysql> select name,id from websites where id = 1;
+------+----+
| name | id |
+------+----+
| Shef | 1 |
+------+----+
1 row in set (0.00 sec)
mysql> select name from websites where id = 1;
+------+
| name |
+------+
| Shef |
+------+
1 row in set (0.00 sec)
对于第一个查询：
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000045 |
| checking permissions | 0.000006 |
| Opening tables | 0.000016 |
| System lock | 0.000007 |
| init | 0.000016 |
| optimizing | 0.000006 |
| statistics | 0.000063 |
| preparing | 0.000008 |
| executing | 0.000004 |
| Sending data | 0.000009 |
| end | 0.000003 |
| query end | 0.000002 |
| closing tables | 0.000005 |
| freeing items | 0.000046 |
| logging slow query | 0.000002 |
| cleaning up | 0.000002 |
+----------------------+----------+
对于第二个查询：
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000050 |
| checking permissions | 0.000006 |
| Opening tables | 0.000015 |
| System lock | 0.000008 |
| init | 0.000017 |
| optimizing | 0.000008 |
| statistics | 0.000044 |
| preparing | 0.000008 |
| executing | 0.000002 |
| Sending data | 0.000009 |
| end | 0.000003 |
| query end | 0.000002 |
| closing tables | 0.000005 |
| freeing items | 0.000046 |
| logging slow query | 0.000002 |
| cleaning up | 0.000002 |
+----------------------+----------+
【讨论】：