Question

这是Ryan Frank在forums.mysql.com上提出的问题，我也面临这个问题。

我在SELECT语句的开头有以下内容：

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;但是，我想将使用COUNT（DISTINCT）的变量限制为日期范围。我不能在FROM子句后使用WHERE子句。例如：

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

不起作用，因为它不会给我所有帐户，就像我需要的那样。

我正在寻找类似的东西：

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

P.S。我知道上面的内容不起作用，并且语法选项已用完。

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

您可以将条件放在ON的{{1}}子句中：

LEFT JOIN

与其他表格相同。

COUNT（DISTINCT）子句中的WHERE子句

2 个答案: