我想从id = 4的数据库表中返回字段name。哪个查询更容易让MYSQL处理。
SELECT name, id FROM table WHERE id = 4
或
SELECT name FROM table WHERE id = 4
我想我问的是，将WHERE语句中的字段放在SELECT中是否也是好的做法，即使我不会在php中返回它。
答案 0 :(得分：3)
SELECT name, id FROM table WHERE id = 4
在mysql的结果中也会有id，它会对数据产生很小的影响，因为它只是一个数字，但如果你这样做，你选择其他的东西，如SELECT name, id FROM table WHERE name = "ron"会绝对是一种不好的做法，因为更多的数据正在从数据库服务器传输，因为您已经知道名称“ron”，因为它不是唯一的，它可能有多行。
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)
与
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)
答案 1 :(得分：1)
如果您不需要返回的ID，请不要包含它。如果您需要，请加入它。
答案 2 :(得分：1)
不，这不是好习惯。 SELECT正是您需要的那些领域。
答案 3 :(得分：1)
我怀疑第二种选择是在大型数据库上更快（许多记录> 1M），因为从数据库中取出数据所需的时间更少。
这是在一个小桌子上完成的分析（1187条记录），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 |
+----------------------+----------+
答案 4 :(得分：1)
我想我要问的是，把这个领域放在哪里是好的做法 SELECT中的WHERE语句，即使我不会在php中返回它。
不。这将增加网络开销，因为数据在客户端服务器之间传输。如果您不打算使用这些数据，请不要选择它。
答案 5 :(得分：1)
除非您在结果中需要“id”列，否则没有理由要求它。 MySQL将对查询进行相同的处理。要求它可能会减慢速度，因为MySQL必须向您返回额外的数据，并且您的程序需要解析它。怀疑它对于'id'列很重要，但如果它可能是几个巨大的文本字段。
以下是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)