JOIN on DATEPART month and year is causing extra rows - sql

Question

I have two tables that contain a date field. This date field is one of the JOIN causes that I would like to implement, but I only want to JOIN on the month and year, not the day. The # of records about triple when I attempt to do so. I'm guessing there is something wrong with my query? Or is this even possible? I'm using Postgres
SELECT a.load_date , a.mandt, a.vbeln,a.posnr, a.matnr, b.tfed
FROM tableA a
JOIN tableB b
ON date_part('month'::text, a.erdat) = date_part('month'::text, b.gdatu)
AND date_part('year'::text, a.erdat) = date_part('year'::text, b.gdatu)
EDIT Here is my full code
SELECT a.mandt, a.vbeln,
a.erdat, a.erzet, a.ernam, a.angdt, a.audat, a.vbtyp, a.trvog,
a.auart, a.submi, a.lifsk, a.faksk, a.netwr, a.waerk, a.vkorg, a.vtweg, a.spart,
a.vkgrp, a.vkbur, a.knumv, a.vdatu, a.vprgr, a.kalsm, a.vsbed, a.fkara, a.awahr,
a.bstnk, a.bstdk, a.telf1, a.kunnr, a.stafo, a.stwae, a.aedat, a.kvgr1,a.kvgr2,
a.kvgr3, a.kokrs, a.kkber, a.knkli, a.sbgrp, a.ctlpc, a.cmwae, a.cmfre, a.cmngv,
a.amtbl, a.hityp_pr, a.abrvw, a.vgbel, a.objnr, a.bukrs_vf, a.taxk1,a.xblnr,
a.vgtyp, a.abhod, a.abhov, a.stceg_l, a.landtx, a.fmbdat, a.vsnmr_v, a.handle,
a.yybcawv1, a.yybcawv2, a.yybcawv3, a.yyawv1dat, a.yyawv2dat, a.yybcawvc,
a.kvgr5, a.augru, a.autlf, a.bname, a.bnddt, a.bsark, a.cmnup, a.fiscalper,
a.fiscalyr, a.gwldt, a.ihrez, a.intind, a.intsum, a.rplnr, a.taxk2, a.yybabt,
a.yybemail, a.yybfax, a.yybname, a.yybphone, a.yyexporter, a.yypaypal_id,
a.yysd_projid, a.zone, a.zuonr, a.zz_campaign_id, a.zzedate, a.zzrev_cat_01,
a.zzrev_cat_02, a.zzrev_cat_03, a.zzrev_cat_04, a.zzrev_cat_05, a.zzrev_cat_06,
a.zzrev_cat_07, a.zzrev_cat_08, a.zzsdate, a.mahdt,
CASE
WHEN b.fcurr::text = 'USD'::text THEN a.netwr
WHEN b.fcurr::text = 'JPY'::text AND b.kurst::text = 'M'::text THEN a.netwr * b.ukurs / 10::numeric
WHEN b.fcurr::text = 'KRW'::text AND b.kurst::text = 'M'::text THEN a.netwr * b.ukurs / 10::numeric
WHEN b.kurst::text = 'M'::text THEN a.netwr * b.ukurs
ELSE a.netwr
END AS net_value_trans_currency_netwr
FROM src.sap_vbak a
JOIN src.sap_tcurr b
ON a.waerk::text = b.fcurr::text
AND date_part('MONTH'::text, a.erdat::timestamp with time zone) = date_part('MONTH'::text, b.gdatu::timestamp with time zone)
AND date_part('YEAR'::text, a.erdat::timestamp with time zone) = date_part('YEAR'::text, b.gdatu::timestamp with time zone);
I'm attempting to get currency conversions based off of the dates (month and year only) in each of the tables. Some of the currency conversion are different ( the CASE statement for net_value_trans_currency_netwr field). I am wanting the net_value_trans_currency_netwr field to be a new row that displays the currency conversion in USD. The original table has over 5 million rows. After the joins I end up with way more rows. From what I gather I'm getting a full join. How would I be able to execute what I'm trying to do without the full join creating more than needed rows?

Answer 1

You get duplicate rows as you are INNER JOINING on the month and year which are not unique. This is causing a cross join e.g.
Example Rows with dates
Date Month Year
1 01/01/2014 01 14
2 02/01/2014 01 14
Result of above join has 4 rows not 2!
1) Month from (1) Year from (1)
2) Month from (1) Year from (2)
3) Month from (2) Year from (1)
4) Month from (2) Year from (2)
If you want to avoid this you need something else to include in the join that makes each join unique! Adding the day may help but again if you have more than one date recorded on the same day you will get a duplicate. Have a think what else you could include on the join.

Answer 2

Use date_trunc() to simplify the query:
SELECT a.load_date, a.mandt, a.vbeln,a.posnr, a.matnr, b.tfed
FROM tableA a
JOIN tableB b ON date_trunc('month', a.erdat)
= date_trunc('month', b.gdatu);
Plus, you probably want to restrict the join further. This is a limited cross join resulting in a Cartesian product. If you have 3 rows for March 2014 in a tableA and 4 rows for March 2014 in a tableB, you already produce 12 rows in the result.