MySQL 一对多和多对多查询

Question

I have three tables: Hospitals, Inspections, and Violations. I need to formulate a query that returns all lighting violations with the inspection date, inspectors notes, and hospital name in one row per inspection.

A Hospital can have zero or more inspections. Inspections can have zero or more violations.

This is the query so far:

SELECT h.hospital_name AS Name, i.date AS Date, CONCAT_WS(', ', v.notes) AS Notes
FROM hospital_table h
LEFT JOIN inspection_table i ON h.id = i.hospital_id
INNER JOIN violation_table v ON i.id = v.inspection_id
WHERE v.type = 'Lighting'
ORDER BY i.date DESC;

There are instances where an inspection will have zero or more lighting violations. Essentially I need the result to look like:

| Name              | Date         | Notes                           |
----------------------------------------------------------------------
| Burnaby General   | 18-09-2107   | Bath light..., bed sconce...    |

The result is close but in some cases, the dates are not correct. Can someone suggest a way to achieve what is required?

Answer 1

Use GROUP_CONCAT instead of CONCAT_WS to get all notes per hospital in one row.

SELECT h.hospital_name AS `name`, 
i.date AS `date`, 
GROUP_CONCAT(v.notes) AS Notes
FROM hospital_table h
LEFT JOIN inspection_table i ON h.id = i.hospital_id
LEFT JOIN violation_table v ON i.id = v.inspection_id
WHERE v.type = 'Lighting'
GROUP BY h.hospital_name
ORDER BY i.date DESC;

Note: The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet.

For date i assume there will be multiple inspections per hospital to get the recent inspection date you can use MAX(i.date) and change ORDER BY MAX(i.date) or to get all inspection dates you can apply GROUP_CONCAT on i.date too