Question

我有三个表：业务，检查和违规。我有以下查询：

SELECT b.id, b.name, b.address, b.city, b.state, b.type, i.date
FROM businesses AS b
RIGHT JOIN inspections i ON i.business_id = b.id
RIGHT JOIN (SELECT inspection_id FROM violations) AS v ON v.inspection_id = i.id
WHERE b.type = 'hospital' AND i.date BETWEEN "2019-01-01" AND "2019-01-03";

我想找出结果中返回的明显违规次数（简单，这是行数），检查和业务。我尝试使用COUNT和GROUP BY，但无法使其正常工作。

我需要的是类似的东西

table      | count
------------------
businesses | 10
inspections| 23
violations | 100

任何指针将不胜感激。

Answer 1

您可以这样操作，它将所有计数放在一行中：

SELECT COUNT(DISTINCT b.id) AS businesses, 
       COUNT(DISTINCT i.id) AS inspections,
       COUNT(DISTINCT v.id) AS violations
FROM businesses AS b
INNER JOIN inspections AS i ON b.id = i.business_id AND i.date BETWEEN "2019-01-01" AND "2019-01-03"
LEFT JOIN violations AS v ON i.id = v.inspection_id
WHERE b.type = 'hospital'

使用INNER JOIN仅返回在该日期范围内已检查的企业数。如果要统计所有业务，请使用OUTER JOIN。

使用WHERE子句计算MySQL中多个联接表的DISTINCT行

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