Php MySQL-为什么这个SELECT查询会给出多条记录_Php_Mysql

Php MySQL-为什么这个SELECT查询会给出多条记录

php mysql

Php MySQL-为什么这个SELECT查询会给出多条记录,php,mysql,Php,Mysql,我正在刷新一些MySQL技能（希望如此）我有一个疑问： 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

我正在刷新一些MySQL技能（希望如此）

我有一个疑问：

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

这返回了正确的信息，但我得到了每条记录的多次出现。你知道我错过了什么吗？TIA

在

FROM

-子句中有4个表。因此，您至少需要3个

连接

，以避免数据的多次出现

基本规则：n个表=（n-1）连接

+++++++++++编辑+++++++++++

在您的情况下，它应该如下所示：

/*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

在

FROM

-子句中有4个表。因此，您至少需要3个

连接

，以避免数据的多次出现

基本规则：n个表=（n-1）连接

+++++++++++编辑+++++++++++

在您的情况下，它应该如下所示：

/*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

在

FROM

-子句中有4个表。因此，您至少需要3个

连接

，以避免数据的多次出现

基本规则：n个表=（n-1）连接

+++++++++++编辑+++++++++++

在您的情况下，它应该如下所示：

/*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

在

FROM

-子句中有4个表。因此，您至少需要3个

连接

，以避免数据的多次出现

基本规则：n个表=（n-1）连接

+++++++++++编辑+++++++++++

在您的情况下，它应该如下所示：

/*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

带有连接的查询可能如下所示

SELECT s.name
     , s.stop_id
     , st.trip_id
     , st.departure_time
     , t.route_id
  FROM stops s
  JOIN routes r
    ON r.some_column = something
  JOIN stop_times st
    ON st.stop_id = s.stop_id 
  JOIN trips t
    ON t.some_column = some_other_table.some_column
 WHERE t.route_id = 2 
   AND st.trip_id = 'SCHED255'
 ORDER 
    BY s.stop_id ASC

带有连接的查询可能如下所示

SELECT s.name
     , s.stop_id
     , st.trip_id
     , st.departure_time
     , t.route_id
  FROM stops s
  JOIN routes r
    ON r.some_column = something
  JOIN stop_times st
    ON st.stop_id = s.stop_id 
  JOIN trips t
    ON t.some_column = some_other_table.some_column
 WHERE t.route_id = 2 
   AND st.trip_id = 'SCHED255'
 ORDER 
    BY s.stop_id ASC

带有连接的查询可能如下所示

SELECT s.name
     , s.stop_id
     , st.trip_id
     , st.departure_time
     , t.route_id
  FROM stops s
  JOIN routes r
    ON r.some_column = something
  JOIN stop_times st
    ON st.stop_id = s.stop_id 
  JOIN trips t
    ON t.some_column = some_other_table.some_column
 WHERE t.route_id = 2 
   AND st.trip_id = 'SCHED255'
 ORDER 
    BY s.stop_id ASC

带有连接的查询可能如下所示

SELECT s.name
     , s.stop_id
     , st.trip_id
     , st.departure_time
     , t.route_id
  FROM stops s
  JOIN routes r
    ON r.some_column = something
  JOIN stop_times st
    ON st.stop_id = s.stop_id 
  JOIN trips t
    ON t.some_column = some_other_table.some_column
 WHERE t.route_id = 2 
   AND st.trip_id = 'SCHED255'
 ORDER 
    BY s.stop_id ASC

您缺少几个连接条件。如果有查询，您将使用什么php输出？@Mureinik the WHERE子句不充当连接？@davidelmonte您可以在那里设置连接条件，但不建议这样做（这些称为隐式连接）。但是，您缺少处理

stop_times

和

trips

表的连接条件。而且您也没有路由条件。您缺少几个连接条件。如果有的话，您使用什么php输出查询？@Mureinik the WHERE子句不充当连接？@davidelmonte您可以在那里拥有连接条件，尽管不推荐（这些称为隐式连接）。但是，您缺少处理

stop_times

和

trips

stop_times

和

trips

stop_times

和

trips

表的连接条件。而且您也没有路由条件。谢谢。。我不需要别人来编写我的代码，但是在我的上下文中使用一个连接的示例会非常有用，因为我不知道您的数据模型。所以我不能给你一个确切的解决方案。您能提供一些关于主键和外键约束的信息吗？谢谢。。我不需要别人来编写我的代码，但是在我的上下文中使用一个连接的示例会非常有用，因为我不知道您的数据模型。所以我不能给你一个确切的解决方案。您能提供一些关于主键和外键约束的信息吗？谢谢。。我不需要别人来编写我的代码，但是在我的上下文中使用一个连接的示例会非常有用，因为我不知道您的数据模型。所以我不能给你一个确切的解决方案。您能提供一些关于主键和外键约束的信息吗？谢谢。。我不需要别人来编写我的代码，但是在我的上下文中使用一个连接的示例会非常有用，因为我不知道您的数据模型。所以我不能给你一个确切的解决方案。你能提供一些关于主键和外键约束的信息吗？