SELECTIFNULL((SELECTCOUNT(*)FROMusersWHEREage>30ANDgender='male'),NULL); 1. 在上面的查询语句中,我们使用了子查询来执行 COUNT 操作,并将结果作为 IFNULL 函数的第一个参数。如果子查询返回的结果为 NULL,即 COUNT 为 0,那么 IFNULL 函数会返回第二个参数,即 NULL。否则,
让它count为0的记录也显示出来,做法如下: SELECT use_city, COUNT(CASE WHEN os='Windows 2003 Std'THEN 1 ELSE NULL END) FROM data GROUP BY use_city
COUNT函数是 SQL 中的聚合函数之一,用来统计所选字段中非 NULL 值的数量。其基本语法如下: SELECTCOUNT(column_name)FROMtable_name; 1. 当我们只需要计算记录的总数时,可以使用: SELECTCOUNT(*)FROMtable_name; 1. 二、排除字段等于0的统计 在实际业务中,我们可能需要过滤掉某些记录,例如,字段值等于0的情况。...
1、首先在桌面上,点击“ManagementStudio”图标。2.然后在窗口中,单击左上角的“newquery”选项。3.然后在这个窗口中,输入SQL语句“selectISNULL(count(*),0)fromtest1wherename='e'当count为0时。”4.然后,在窗口中,单击工具栏中的“执行”图标。5.最后,在此窗口中,当count为0时显示成功。
SELECT p.id AS product_id, p.name AS product_name, COUNT(o.product_id) AS order_count FROM products p LEFT JOIN orders o ON p.id = o.product_id GROUP BY p.id, p.name ORDER BY product_name; 步骤3: 解释查询语句 LEFT JOIN orders o ON p.id = o.product_id:这确保了products表...
现在表A数据id name0 aaa1 bbb2 ccc 表B数据id detailAlpha0 one0 two0 three2 one2 two 表C数据id detailBeta0 one0 two1 one1 two A.id=B.id & A.id=C.id关联查询语句count如何写能够返回0 我想select的cou...
sql mysql> SELECT COUNT(*) FROM student;This optimization only applies to MyISAM tables, because an exact row count is stored for this storage engine and can be accessed very quickly.COUNT(1) is only subject to the same optimization if the first column is defined as NOT NULL. ...
mysql> select count(*) from api_runtime_log;+---+| count(*) |+---+| 5718952 |+---+1 row in set (42.95 sec)我们再去看下表结构,看上去貌似也挺正常的~存在主键,表引擎也是 InnoDB,字符集也没问题。CREATE TABLE `api_runtime_log_copy` ( `BelongXiaQuCode` varchar(50) DEFAULT ...
select count(1) from t_order;如果表里只有主键索引,没有二级索引时。那么,InnoDB 循环遍历聚簇索引...
select count(*) from table where is_active is null; select count(1) from table where is_active is null; 特例: select count('') from table; 返回表的记录数 select count(0) from table; 返回表的记录数 select count(null) from table; 返回0...