MYSQL在選擇語句中的字段呢？

Question

我想從id = 4的數據庫表中返回字段name。MYSQL處理哪個查詢更容易。

SELECT name, id FROM table WHERE id = 4 

或

SELECT name FROM table WHERE id = 4 

我想我要問，如果它是很好的做法，把字段從WHERE聲明SELECT以及即使我不會在PHP返回。

Answer 1

SELECT name, id FROM table WHERE id = 4 

也有從MySQL結果的ID，如果你這樣做，你正在選擇其他的東西，這將使相差無幾的數據，因爲它僅僅是一個數字然而作爲SELECT name, id FROM table WHERE name = "ron"肯定會是一個不好的做法，因爲更多數據正在從數據庫服務器傳輸，因爲您已經知道名稱「ron」，並且因爲它不是唯一的，所以它可能有多個行。

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) 

Answer 2

如果您不需要返回的ID，請不要包含它。如果你需要它，請包括它。

Answer 3

不，這不是很好的做法。 SELECT恰恰是你需要的那些領域。

Answer 4

我懷疑第二個選擇，在大數據庫（許多記錄> 1M）上更快，因爲從數據庫中提取數據所需的時間更少。

---

這裏是一個小桌子（1187條記錄）做了分析，ID是一個主鍵：

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) 

對於第一個查詢：

+----------------------+----------+ 
| 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 | 
+----------------------+----------+ 

對於第二個查詢：

+----------------------+----------+ 
| 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 | 
+----------------------+----------+ 

Answer 5

我想我是問是否很好的做法是將SELECT語句中的 WHERE語句中的字段放入，即使我不會在PHP中返回它。

沒有。這會在客戶端服務器之間傳輸數據時增加網絡開銷。如果您不打算使用這些數據，請不要選擇它。

Answer 6

除非您在結果中需要'id'列，否則沒有理由要求它。 MySQL會將查詢視爲相同。詢問它可能會減慢速度，因爲MySQL必須將額外的數據返回給您，並且您的程序需要解析它。懷疑這對'id'欄很重要，但如果它是幾個巨大的文本字段，它可能會。

以下是MySQL對我的簡單表上的兩個查詢（在id字段中有一個索引）的看法。

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)