Mysql中count()函數的一般用法是統計字段非空的記錄數,所以可以利用這個特點來進行條件統計,注意這裏如果字段是NULL就不會統計,但是false是會被統計到的,記住這一點,我們接下來看看幾種常見的條件統計寫法。
測試環境
Windows 10
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準備工作
新建一個Mysql數據表a,包含id和num兩個字段
mysql> create table a(id int, num int);
Query OK, 0 rows affected (0.04 sec)
插入測試數據,爲了看count()函數的效果,我們插入兩個空數據
mysql> insert into a values (1,100),(2,200),(3,300),(4,300),(8,null),(9,null);
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
查詢表a中的數據,與後面的統計做比較
mysql> select * from a;
| id | num |
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 4 | 300 |
| 8 | NULL |
| 9 | NULL |
6 rows in set (0.09 sec)
調用count()函數看效果,如果使用count(*)會查詢出所有的記錄數,但如果使用count(num)發現只有4條數據,num爲NULL的記錄並沒有統計上
mysql> select count(*) from a;
| count(*) |
| 6 |
1 row in set (0.03 sec)
mysql> select count(num) from a;
| count(num) |
| 4 |
1 row in set (0.04 sec)
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count()函數中使用條件表達式加or null來實現,作用就是當條件不滿足時,函數變成了count(null)不會統計數量
mysql> select count(num > 200 or null) from a;
| count(num > 200 or null) |
| 2 |
1 row in set (0.22 sec)
count()函數中使用if表達式來實現,當條件滿足是表達式的值爲非空,條件不滿足時表達式值爲NULL;
mysql> select count(if(num > 200, 1, null)) from a;
| count(if(num > 200, 1, null)) |
| 2 |
1 row in set (0.05 sec)
count()函數中使用case when表達式來實現,當條件滿足是表達式的結果爲非空,條件不滿足時無結果默認爲NULL;
mysql> select count(case when num > 200 then 1 end) from a;
| count(case when num > 200 then 1 end) |
| 2 |
1 row in set (0.07 sec)
總結
使用count()函數實現條件統計的基礎是對於值爲NULL的記錄不計數,常用的有以下三種方式,假設統計num大於200的記錄
select count(num > 200 or null) from a;
select count(if(num > 200, 1, null)) from a
select count(case when num > 200 then 1 end) from a