创建两个联接到同一个表

Question

From Scheduled I need to make two joins to country, two joins to state and two joins to city.

My Tables

Schedule
Destination Country ID  
Destination City ID
Destination State ID
Current Country ID
Current City ID
Current State ID
-------------------
Country
ID
Country
-------------------
State
ID
Statename
Countryid
-------------------
City
ID
City
Stateid
Countryid

I need to read the data from schedule table and list it using the country, state and city names, which are stored in the country, state and city tables, and linked by the ID fields. I have the first join to each table working, but when I try to second the query fails saying “ Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given ”.

What works…

$query="SELECT schedule.username, schedule.ret, country.country, state.statename, city.city, schedule.business, schedule.pleasure 
FROM schedule 
JOIN country 
ON schedule.dest_country=country.id
JOIN state
ON schedule.dest_state=state.id
JOIN city
ON schedule.dest_city=city.id";

What does not...

$query="SELECT schedule.username, schedule.ret, country.country, state.statename, city.city, schedule.business, schedule.pleasure, country.country 
FROM schedule 
JOIN country 
ON schedule.dest_country=country.id
JOIN state
ON schedule.dest_state=state.id
JOIN city
ON schedule.dest_city=city.id
JOIN country 
ON schedule.cur_country=country.id";

Thanks

Answer 1

Use aliases on your join tables to separate the duplicate table joins:

$query="SELECT schedule.username, schedule.ret, destcountry.country, state.statename,          city.city, schedule.business, schedule.pleasure, curcountry.country  
FROM schedule 
JOIN country destcountry
ON schedule.dest_country=destcountry.id
JOIN state
ON schedule.dest_state=state.id
JOIN city
ON schedule.dest_city=city.id
JOIN country curcountry
ON schedule.cur_country=curcountry.id";

And to clarify the result set you can name your resulting columns:

$query="SELECT schedule.username, schedule.ret, destcountry.country as destcountry, state.statename, city.city, schedule.business, schedule.pleasure, curcountry.country as currentcountry

Answer 2

try this SQL bro!

$query="SELECT * FROM schedule SC
LEFT JOIN country C ON C.id = SC.dest_country
LEFT JOIN state ST ON ST.id = SC.dest_state
LEFT JOIN city Ci ON Ci.id = SC.dest_city
LEFT JOIN country C2 ON C2.id = SC.cur_country";