JOIN the same table on two columns - sql

Question

I use JOINs to replace country and product IDs in import and export data with actual country and products names stored in separate tables. In the data source table (data), there are two columns with country IDs, for origin and destination, both of which I am replacing with country names.
The code I have come up with refers to the country_names table twice – as country_names, and country_names2, – which doesn’t seem to be very elegant. I expected to be able to refer to the table just once, by a single name. I would be grateful if someone pointed me to a more elegant and maybe more efficient way to achieve the same result.
SELECT
country_names.name AS origin,
country_names2.name AS dest,
product_names.name AS product,
SUM(data.export_val) AS export_val,
SUM(data.import_val) AS import_val
FROM
OEC.year_origin_destination_hs92_6 AS data
JOIN
OEC.products_hs_92 AS product_names
ON
data.hs92 = product_names.hs92
JOIN
OEC.country_names AS country_names
ON
data.origin = country_names.id_3char
JOIN
OEC.country_names AS country_names2
ON
data.dest = country_names2.id_3char
WHERE
data.year > 2012
AND data.export_val > 1E8
GROUP BY
origin,
dest,
product
The table to convert product IDs to product names has 6K+ rows. Here is a small sample:
id hs92 name
63215 3215 Ink
2130110 130110 Lac
21002 1002 Rye
2100200 100200 Rye
52706 2706 Tar
20902 902 Tea
42203 2203 Beer
42302 2302 Bran
178703 8703 Cars
The table to convert country IDs to country names (which is the table I have to JOIN on twice) has 264 rows for all countries in the world. (id_3char is the column used.) Here is a sample:
id id_3char name
euchi chi Channel Islands
askhm khm Cambodia
eublx blx Belgium-Luxembourg
eublr blr Belarus
eumne mne Montenegro
euhun hun Hungary
asmng mng Mongolia
nabhs bhs Bahamas
afsen sen Senegal
And here is a sample of data from the import and export data table with a total of 205M rows that has the two columns origin and dest that I am making a join on:
year origin dest hs92 export_val import_val
2009 can isr 300410 2152838.47 3199.24
1995 chn jpn 590190 275748.65 554154.24
2000 deu gmb 100610 1573508.44 1327.0
2008 deu jpn 540822 10000.0 202062.43
2010 deu ukr 950390 1626012.04 159423.38
2006 esp prt 080530 2470699.19 125291.33
2006 grc ind 844859 8667.0 3182.0
2000 ltu deu 630399 6018.12 5061.96
2005 usa zaf 290219 2126216.52 34561.61
1997 ven ecu 281122 155347.73 1010.0

Answer 1

I think you already have it done such that it can be considered good enough to just use as is :o)
Meantime, If for some reason you really-really want to avoid two joins on that country table - what you can do is to materialize below select statement into let's say `OEC.origin_destination_pairs` table
SELECT
o.id_3char o_id_3char,
o.name o_name,
d.id_3char d_id_3char,
d.name d_name
FROM `OEC.country_names` o
CROSS JOIN `OEC.country_names` d
Then you can just join on that new table as below
SELECT
country_names.o_name AS origin,
country_names.d_name AS dest,
product_names.name AS product,
SUM(data.export_val) AS export_val,
SUM(data.import_val) AS import_val
FROM OEC.year_origin_destination_hs92_6 AS data
JOIN OEC.products_hs_92 AS product_names
ON data.hs92 = product_names.hs92
JOIN OEC.origin_destination_pairs AS country_names
ON data.origin = country_names.o_id_3char
AND data.dest = country_names2.d_id_3char
WHERE data.year > 2012
AND data.export_val > 1E8
GROUP BY
origin,
dest,
product
The motivation behind above is cost of storing and querying in your particular case
Your `OEC.country_names` table is just about 10KB in size
Each time you query it you pay as if it is 10MB (Charges are rounded to the nearest MB, with a minimum 10 MB data processed per table referenced by the query, and with a minimum 10 MB data processed per query.)
So, if you will materialize above mentioned table - it will still be less than 10MB so no difference in querying charges
Similar situation with storing that table - no visible changes in charges
You can check more about pricing here