COUNT（DISTINCT）子句中的WHERE子句

Question

这是一个由Ryan Frank在forums.mysql.com上提出的问题，我也正在面对这个问题。

我已经在我的SELECT语句的开头如下：

SELECT accounts.id, accounts.company, accounts.first, accounts.last, 
COUNT(DISTINCT accounts_log.login_time) AS visits, 
COUNT(DISTINCT accounts_log.ip_address) AS visitors, 
COUNT(DISTINCT documents_log.access_time) AS docs, 
MAX(accounts_log.login_time) AS login_time 
FROM accounts 

这将返回我需要的所有变量;不过，我想将使用COUNT（DISTINCT）的变量限制在日期范围内。我无法在FROM子句之后使用WHERE子句。例如：

FROM accounts 
WHERE accounts_log.login_time >='$search_from' AND accounts_log.login_time <='$search_to' 

将无法​​正常工作，因为它不会给我像我需要的所有帐户。

我在寻找类似：

COUNT(DISTINCT accounts_log.login_time WHERE accounts_log.login_time >='$search_from' AND accounts_log.login_time <='$search_to') AS visits 

附：我知道上述不起作用，并已用完语法选项。

Answer 1

SELECT accounts.id, accounts.company, accounts.first, accounts.last, 
COUNT(DISTINCT case when accounts_log.login_time >='$search_from' AND accounts_log.login_time <='$search_to' then accounts_log.login_time else null end) AS visits, 
COUNT(DISTINCT case when accounts_log.login_time >='$search_from' AND accounts_log.login_time <='$search_to' then accounts_log.ip_address else null end) AS visitors, 
COUNT(DISTINCT case when accounts_log.login_time >='$search_from' AND accounts_log.login_time <='$search_to' then documents_log.access_time else null end) AS docs, 
MAX(accounts_log.login_time) AS login_time 
FROM accounts 

Answer 2

你可以把条件的LEFT JOINON条款：

SELECT a.id, a.company, a.first, a.last, 
COUNT(DISTINCT al.login_time) AS visits 
FROM accounts a 
LEFT JOIN accounts_log al ON (al.account_id = a.id AND 
al.login_time BETWEEN '$search_from' AND '$search_to') 

和同其他表。