sql - 如果使用 where 子句时数据为空，则将数据计为零

Question

现在我有这个查询:

SELECT 
opp.name as name,
count(log.stage_id) as stage_count
FROM 
crm_lead as opp LEFT OUTER JOIN crm_lead_stage_log as log ON (opp.id = log.opportunity_id)
GROUP BY
name

它输出这个结果:

name  | stage_count |
name1 | 2
name2 | 1
name3 | 0 

它输出我需要的。但是如果我给它加上任何条件，它就会跳过计数为零的行，我需要能够看到这些行。例如，如果我写这个查询:

SELECT 
opp.name as name,
count(log.stage_id) as stage_count
FROM 
crm_lead as opp LEFT OUTER JOIN crm_lead_stage_log as log ON (opp.id = log.opportunity_id)
WHERE WHEN log.create_date > '2014-01-28 08:49:03'
GROUP BY
name

然后它输出这个:

name  | stage_count |
name1 | 1

它会正确计算该时间间隔内的现有阶段数，但会跳过该时间间隔内不存在的阶段数的行。我怎样才能让它像这样输出(在那个例子中，只有第一行的一个阶段被计算在新查询的那个时间间隔内，对于其他行，它计数为零，因为它不存在):

name  | stage_count |
name1 | 1
name2 | 0
name3 | 0 

这样可以吗？附言如果需要更多信息，例如将此查询示例放在网上查看，只需发表评论，我会更新我的答案)。

Answer 1

如果要在结果为空时返回零​​，可以使用命令 COALESCE。

SELECT 
opp.name AS name,
COALESCE(COUNT(log.stage_id),0) AS stage_count
FROM 
crm_lead AS opp LEFT OUTER JOIN crm_lead_stage_log AS log ON (opp.id = log.opportunity_id)
GROUP BY name

当计数为空值时返回“0”。