I am using the following command to get all refseq genes from UCSC:/usr/bin/mysql --user=genomep --password=password --host=genome-mysql.cse.ucsc.edu \    -A -D hg38 -e 'select concat(t.name, ., i.version) name, \    k.locusLinkId as EntrezId, t.chrom, t.strand, t.txStart, \    t.txEnd, t.cdsStart, t.cdsEnd, t.exonCount, t.exonStarts, \    t.exonEnds, t.score, t.name2 from refGene t join hgFixed.gbCdnaInfo i \    on t.name = i.acc join hgFixed.refLink k on t.name = k.mrnaAcc'That returns data in the following format (showing the 1st 5 lines):+-------------+-----------+-------+--------+---------+-------+----------+--------+-----------+--------------------+--------------------+-------+-----------+| name        | EntrezId  | chrom | strand | txStart | txEnd | cdsStart | cdsEnd | exonCount | exonStarts         | exonEnds           | score | name2     |+-------------+-----------+-------+--------+---------+-------+----------+--------+-----------+--------------------+--------------------+-------+-----------+| NR_046018.2 | 100287102 | chr1  | +      |   11873 | 14409 |    14409 |  14409 |         3 | 11873,12612,13220, | 12227,12721,14409, |     0 | DDX11L1   || NR_106918.1 | 102466751 | chr1  | -      |   17368 | 17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-1 || NR_107062.1 | 102465909 | chr1  | -      |   17368 | 17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-2 || NR_107063.1 | 102465910 | chr1  | -      |   17368 | 17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-3 || NR_128720.1 | 103504738 | chr1  | -      |   17368 | 17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-4 |+-------------+-----------+-------+--------+---------+-------+----------+--------+-----------+--------------------+--------------------+-------+-----------+I also want to find the accession of the canonical transcript of each of the genes returned by the command above. Those seem to be stored in the knownCanonical table:$ /usr/bin/mysql --user=genomep --password=password --host=genome-mysql.cse.ucsc.edu -A -D hg38 -e 'select * from knownCanonical limit 5'+-------+------------+-----------+-----------+------------+--------------------+| chrom | chromStart | chromEnd  | clusterId | transcript | protein            |+-------+------------+-----------+-----------+------------+--------------------+| chrX  |  100628669 | 100636806 |         1 | uc004ega.3 | ENSG00000000003.14 || chrX  |  100584801 | 100599885 |         2 | uc004efy.5 | ENSG00000000005.5  || chr20 |   50934866 |  50958550 |         3 | uc002xvw.2 | ENSG00000000419.12 || chr1  |  169853073 | 169893959 |         4 | uc001ggs.5 | ENSG00000000457.13 || chr1  |  169795048 | 169854080 |         5 | uc001ggp.4 | ENSG00000000460.16 |+-------+------------+-----------+-----------+------------+--------------------+However, there seems to be no obvious way to link the knownCanonical table to the refGene, hgFixed.gbCdnaInfo and hgFixed.refLink tables used above. So, how can I modify my 1st query (or, if necessary write a new one) so that my results also include the accession of the gene's canonical transcript? 
Get canonical transcript from UCSC
public databases;identifiers;ucsc
You can link them with the kgXref table, since kgXref.refseq == refGene.name and kgXref.kgID == knownCanonical.transcript. Since it seems that knownCanonical.transcript is what you want anyway, you don't even need to join on it:mysql --user=genomep --password=password --host=genome-mysql.cse.ucsc.edu \-A -D hg38 -e 'select concat(t.name, ., i.version) name, x.kgID, \k.locusLinkId as EntrezId, t.chrom, t.strand, t.txStart, \t.txEnd, t.cdsStart, t.cdsEnd, t.exonCount, t.exonStarts, \t.exonEnds, t.score, t.name2 from refGene t join (hgFixed.gbCdnaInfo i, hgFixed.refLink k, kgXref x) \on (t.name = i.acc and t.name = k.mrnaAcc and t.name = x.refseq) limit 10'The output is then:+-------------+------------+-----------+-------+--------+---------+--------+----------+--------+-----------+--------------------+--------------------+-------+------------+| name        | kgID       | EntrezId  | chrom | strand | txStart | txEnd  | cdsStart | cdsEnd | exonCount | exonStarts         | exonEnds           | score | name2      |+-------------+------------+-----------+-------+--------+---------+--------+----------+--------+-----------+--------------------+--------------------+-------+------------+| NR_106918.1 | uc031tla.1 | 102466751 | chr1  | -      |   17368 |  17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-1  || NR_107062.1 | uc031tlm.1 | 102465909 | chr1  | -      |   17368 |  17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-2  || NR_107063.1 | uc032cta.1 | 102465910 | chr1  | -      |   17368 |  17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-3  || NR_128720.1 | uc032dmn.1 | 103504738 | chr1  | -      |   17368 |  17436 |    17436 |  17436 |         1 | 17368,             | 17436,             |     0 | MIR6859-4  || NR_036051.1 | uc031tlb.1 | 100302278 | chr1  | +      |   30365 |  30503 |    30503 |  30503 |         1 | 30365,             | 30503,             |     0 | MIR1302-2  || NR_036266.1 | uc033cjs.1 | 100422831 | chr1  | +      |   30365 |  30503 |    30503 |  30503 |         1 | 30365,             | 30503,             |     0 | MIR1302-9  || NR_036267.1 | uc032csz.1 | 100422834 | chr1  | +      |   30365 |  30503 |    30503 |  30503 |         1 | 30365,             | 30503,             |     0 | MIR1302-10 || NR_036268.1 | uc032hiw.1 | 100422919 | chr1  | +      |   30365 |  30503 |    30503 |  30503 |         1 | 30365,             | 30503,             |     0 | MIR1302-11 || NR_026822.1 | uc001aak.4 |    654835 | chr1  | -      |   34610 |  36081 |    36081 |  36081 |         3 | 34610,35276,35720, | 35174,35481,36081, |     0 | FAM138C    || NR_106918.1 | uc031tla.1 | 102466751 | chr1  | -      |  187890 | 187958 |   187958 | 187958 |         1 | 187890,            | 187958,            |     0 | MIR6859-1  |+-------------+------------+-----------+-------+--------+---------+--------+----------+--------+-----------+--------------------+--------------------+-------+------------+If you want the protein ID then join knownCanonical on x.kgID.