Document the fact that SQLite allows NULL values in PRIMARY KEY columns

and the fact that we might design to change this in the future.
Ticket #518. (CVS 3373)

FossilOrigin-Name: b99d845ef4776595022c6fb855e582cfe6ef2f9f

manifest

@@ -1,5 +1,5 @@
-C Improved\sbuilt-in\shelp\sin\sthe\scommand-line\sshell.\s\sEnable\sthe\nfulltext\ssearch\sextension\sin\sthe\sTCL\sinterface.\s(CVS\s3372)
-D 2006-08-29T12:04:19
+C Document\sthe\sfact\sthat\sSQLite\sallows\sNULL\svalues\sin\sPRIMARY\sKEY\scolumns\nand\sthe\sfact\sthat\swe\smight\sdesign\sto\schange\sthis\sin\sthe\sfuture.\nTicket\s#518.\s(CVS\s3373)
+D 2006-08-29T13:08:38
 F Makefile.in 8e7f9ecebab2c6e0f3db20ff129a8f9405ab64f8
 F Makefile.linux-gcc 2d8574d1ba75f129aba2019f0b959db380a90935
 F README 9c4e2d6706bdcc3efdd773ce752a8cdab4f90028
@@ -366,7 +366,7 @@ F www/fullscanb.gif f7c94cb227f060511f8909e10f570157263e9a25
 F www/index-ex1-x-b.gif f9b1d85c3fa2435cf38b15970c7e3aa1edae23a3
 F www/index.tcl 7b9c5747a838501dace958131755651d6ca31adb
 F www/indirect1b1.gif adfca361d2df59e34f9c5cac52a670c2bfc303a1
-F www/lang.tcl 2cdc7d6a897d8f9da20989985d76326e76afe6c4
+F www/lang.tcl 529f5f0f44b55f26cdb66c6547a86b1332e1144a
 F www/lockingv3.tcl f59b19d6c8920a931f096699d6faaf61c05db55f
 F www/mingw.tcl d96b451568c5d28545fefe0c80bee3431c73f69c
 F www/nulls.tcl ec35193f92485b87b90a994a01d0171b58823fcf
@@ -388,7 +388,7 @@ F www/tclsqlite.tcl bb0d1357328a42b1993d78573e587c6dcbc964b9
 F www/vdbe.tcl 87a31ace769f20d3627a64fa1fade7fed47b90d0
 F www/version3.tcl 890248cf7b70e60c383b0e84d77d5132b3ead42b
 F www/whentouse.tcl 97e2b5cd296f7d8057e11f44427dea8a4c2db513
-P 58006e38af760b53cf72bf127d7c7b8a619a1282
-R 29f7ed7a0d966745a2c83716a5630c77
+P 9763b4bfd5f0579f9fb821899ffa8bfe650054d7
+R e72221fee1f8d400d3a9ceae8a3d3fc0
 U drh
-Z 1df6398b1b50a7becef0bc2a58ad5ecd
+Z 7758aacd5468f280d9ccb11e2061d137

manifest.uuid

@@ -1 +1 @@
-9763b4bfd5f0579f9fb821899ffa8bfe650054d7
+b99d845ef4776595022c6fb855e582cfe6ef2f9f

www/lang.tcl

@@ -1,7 +1,7 @@
 #
 # Run this Tcl script to generate the lang-*.html files.
 #
-set rcsid {$Id: lang.tcl,v 1.115 2006/08/16 22:58:50 drh Exp $}
+set rcsid {$Id: lang.tcl,v 1.116 2006/08/29 13:08:38 drh Exp $}
 source common.tcl
 
 if {[llength $argv]>0} {
@@ -564,6 +564,18 @@ that B-Tree keys are automatically generated.  Additional detail
 on automatic B-Tree key generation is available
 <a href="autoinc.html">separately</a>.</p>
 
+<p>According to the SQL standard, PRIMARY KEY should imply NOT NULL.
+Unfortunately, due to a long-standing coding oversight, this is not 
+the case in SQLite.  SQLite allows NULL values
+in a PRIMARY KEY column.  We could change SQLite to conform to the
+standard (and we might do so in the future), but by the time the
+oversight was discovered, SQLite was in such wide use that we feared
+breaking legacy code if we fixed the problem.  So for now we have
+chosen to contain allowing NULLs in PRIMARY KEY columns.
+Developers should be aware, however, that we may change SQLite to
+conform to the SQL standard in future and should design new programs
+accordingly.</p>
+
 <p>If the "TEMP" or "TEMPORARY" keyword occurs in between "CREATE"
 and "TABLE" then the table that is created is only visible
 within that same database connection