Comment fixes and improvements in sqlite3ProcessJoin().

FossilOrigin-Name: 1118655f4d58da1273e83954c80d8bc17fa3bde7a39e81cb9947d59894d5ab93

manifest

@@ -1,5 +1,5 @@
-C Improved\stracking\sof\snested\sSELECT\sobjects\sused\sto\simplement\nparenthensized\sFROM\sterms.
-D 2022-04-20T15:07:39.807
+C Comment\sfixes\sand\simprovements\sin\ssqlite3ProcessJoin().
+D 2022-04-20T16:26:22.293
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 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -552,7 +552,7 @@ F src/printf.c 05d8dfd2018bc4fc3ddb8b37eb97ccef7abf985643fa1caebdcf2916ca90fa32
 F src/random.c 097dc8b31b8fba5a9aca1697aeb9fd82078ec91be734c16bffda620ced7ab83c
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 F src/rowset.c ba9515a922af32abe1f7d39406b9d35730ed65efab9443dc5702693b60854c92
-F src/select.c df2bea1b4c6a5456f0f9892a633d59c2e2a3be0179ecd0c803bca75ddc1fd28d
+F src/select.c 88f00d5b9b399f96cabe23a97cfc2be41c34696b057e4563b7e7bdb68c5b2a17
 F src/shell.c.in ecff6f4ab0e8e25acd578b3eb27ac0bad4d48643c160259a277151d6e9d412e1
 F src/sqlite.h.in 2a35f62185eb5e7ecc64a2f68442b538ce9be74f80f28a00abc24837edcf1c17
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -1950,8 +1950,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
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 U drh
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+Z 8b4e17289b78c9908d49197f73d337c0
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-0da2232624571f4020c05d775ea518514d748fba8dacd4caba2e2e6ed1ae399f
+1118655f4d58da1273e83954c80d8bc17fa3bde7a39e81cb9947d59894d5ab93

src/select.c

@@ -456,15 +456,22 @@ static void unsetJoinExpr(Expr *p, int iTable){
 
 /*
 ** This routine processes the join information for a SELECT statement.
-** ON and USING clauses are converted into extra terms of the WHERE clause.
-** NATURAL joins also create extra WHERE clause terms.
+**
+**   *  A NATURAL join is converted into a USING join.  After that, we
+**      do not need to be concerned with NATURAL joins and we only have
+**      think about USING joins.
+**
+**   *  ON and USING clauses result in extra terms being added to the
+**      WHERE clause to enforce the specified constraints.  The extra
+**      WHERE clause terms will be tagged with EP_FromJoin or
+**      EP_InnerJoin so that we know that they originated in ON/USING.
 **
 ** The terms of a FROM clause are contained in the Select.pSrc structure.
 ** The left most table is the first entry in Select.pSrc.  The right-most
 ** table is the last entry.  The join operator is held in the entry to
-** the left.  Thus entry 0 contains the join operator for the join between
+** the right.  Thus entry 1 contains the join operator for the join between
 ** entries 0 and 1.  Any ON or USING clauses associated with the join are
-** also attached to the left entry.
+** also attached to the right entry.
 **
 ** This routine returns the number of errors encountered.
 */
@@ -551,8 +558,8 @@ static int sqlite3ProcessJoin(Parse *pParse, Select *p){
         if( (pSrc->a[0].fg.jointype & JT_LTORJ)!=0 ){
           /* This branch runs if the query contains one or more RIGHT or FULL
           ** JOINs.  If only a single table on the left side of this join
-          ** contains the zName column, then this routine is branch is
-          ** a no-op.  But if there are two or more tables on the left side
+          ** contains the zName column, then this branch is a no-op.
+          ** But if there are two or more tables on the left side
           ** of the join, construct a coalesce() function that gathers all
           ** such tables.  Raise an error if more than one of those references
           ** to zName is not also within a prior USING clause.