SQL COUNT 条记录在表 2 JOINS 掉

Question

使用 MySQL，我有三个表:

项目:

ID  name
1   "birthday party"
2   "soccer match"
3   "wine tasting evening"
4   "dig out garden"
5   "mountainbiking"
6   "making music"

批处理:

ID  projectID  templateID  when
1   1          1            7 days before
2   1          1            1 day  before
3   4          2           21 days before
4   4          1            7 days before
5   5          1            7 days before
6   3          5            7 days before
7   3          3           14 days before
8   5          1           14 days before

模板:

ID  name  message
1   inf1  "Hi, I'd like to invite ..."
2   for1  "Dear Sir, Madam, ..."
3   can1  "Can you please ..."
4   inf2  "Would you like to ..."
5   all1  "To all dear friends ..."
6   inf3  "Does any of you guys ..."

我想显示一个模板表和使用它们的项目数。 因此，结果应该是(已更新!):

templateName  projectCount
inf1          3
for1          1
can1          1
inf2          0
all1          1
inf3          0

我尝试过使用各种 JOIN 的各种 SQL 查询，但我想这对我来说太复杂了。是否可以使用单个 SQL 语句获得此结果？

Answer 1

SELECT t.name templateName, COUNT(DISTICT b.projectID) projectCount
FROM templates t
LEFT OUTER JOIN batches b ON t.ID = b.templateID
GROUP BY t.ID, t.name
ORDER BY t.ID