php - MySQL - 为什么这个 SELECT 查询会给出多条记录

Question

我正在刷新一些 MySQL 技能(希望如此)。

我有这个问题:

SELECT stops.name, 
    stops.stop_id,  
    stop_times.trip_id,
    stop_times.departure_time,
    trips.route_id
FROM stops, routes, stop_times, trips
WHERE stops.stop_id = stop_times.stop_id AND trips.route_id = 2 AND stop_times.trip_id = 'SCHED255'
ORDER BY stops.stop_id ASC

这会返回正确的信息，但我得到了每条记录的多次出现。有什么想法我想念的吗？时间差

Answer 1

您在 FROM 子句中有 4 个表。因此，您至少需要 3 个 JOINS 以避免多次出现数据。

基本规则:n 个表 = (n-1) 个 JOINS。

+++++++++++编辑++++++++++++

在你的情况下它应该是这样的:

/*WITH EXCPLICIT JOINS*/
SELECT stops.name, stops.stop_id, stop_times.trip_id, stop_times.departure_time, trips.route_id
FROM stops s JOIN routes r ON s.col_name = r.col_name /*col_name is the name of the column that is defined as Contraint between these two*/
             JOIN stop_times st ON st.stop_id = s.stop_id
             JOIN trips t ON t.route_id = r.route_id
    AND t.route_id = 2 
    AND st.trip_id = 'SCHED255'
ORDER BY s.stop_id ASC

/*THE SAME THING WITH IMPLICIT JOINS*/
SELECT stops.name, stops.stop_id, stop_times.trip_id, stop_times.departure_time, trips.route_id
FROM stops AS s, routes AS r, stop_times AS st, trips AS t
WHERE s.col_name = r.col_name /*JOIN of stops and trips*/
    AND st.stop_id = s.stop_id /*JOIN of stop_times and stops*/
    AND t.route_id = r.route_id /*JOIN of trips and rutes*/
    AND t.route_id = 2 /*Other condition*/
    AND st.trip_id = 'SCHED255' /*Other condition*/
ORDER BY s.stop_id ASC