从4个表中获取数据mysql

Question

I have 4 tables as below

Table1: restos Structure:

resu_id    resu_name     resu_address
-------------------------------------
   1      ABC             Exapmple
   2      DEF             Example
   3      GHD             Example

Table2: foodtype Structure:

id    typename
---------------
 12    Indian
 23    Punjabi

Table3: resto_foodtypes Structure:

resu_id    foodty_id
--------------------
  1          12
  2          23
  3          12

Table4: discnts Structure:

id    resu_id    amt_dscPer(%age discount)
---------------------------
 19     1          15
 20     2          25

Now i want to display the restaurant along with discounts available for the restauarant. Currently restaurants are getting displayed but for the restaurant not present in discnts table are returning null values from discnts table.

below is the query that m using

SELECT * from `restos` r  join resto_foodtypes rf on rf.resu_id = r.resu_id 
join foodtype f on rf.foodty_id = f.id left join discnts dcfm on 
r.resu_id= dcfm.resu_id where true;

I want that the restaurants that are not present in discnts table should not be included in resultset. For eg resu_id=3 is not present in discnts table.

Answer 1

To exclude results that have no entry in discnts table, use a INNER JOIN instead of a LEFT JOIN .

To include these results, but have a 0 displayed (instead of the defaults NULL ), you can use IFNULL(expr, 0) function:

SELECT 
  r.resu_name AS Name, 
  f.typename AS Food,
  IFNULL(dcfm.amt_dscPer, 0) AS Discount
FROM `restos` r 
JOIN resto_foodtypes rf ON rf.resu_id = r.resu_id 
JOIN foodtype f ON rf.foodty_id = f.id 
LEFT JOIN discnts dcfm ON r.resu_id= dcfm.resu_id;

IFNULL returns the first parameter if it is not null, the second if expr is indeed null.