MySQL compatibility: SUBSTR with a negative start index now works like MySQL.

h2/src/docsrc/help/help.csv

@@ -3096,10 +3096,12 @@ CALL UTF8TOSTRING(STRINGTOUTF8('This is a test'))
 { SUBSTRING | SUBSTR } ( string, startInt [, lengthInt ] )
 ","
 Returns a substring of a string starting at a position.
+If the start index is negative, then the start index is relative to the end of the string.
 The length is optional.
 Also supported is: ""SUBSTRING(string FROM start [FOR length])"".
 ","
-SUBSTR(NAME, 1)
+CALL SUBSTR('[Hello]', 2, 5);
+CALL SUBSTR('Hello World', -5);
 "
 
 "Functions (String)","UTF8TOSTRING","

h2/src/docsrc/html/changelog.html

@@ -18,7 +18,8 @@ Change Log
 <h1>Change Log</h1>
 
 <h2>Next Version (unreleased)</h2>
-<ul><li>When enabling autocommit, the transaction is now committed (as required by the JDBC API).
+<ul><li>MySQL compatibility: SUBSTR with a negative start index now works like MySQL.
+</li><li>When enabling autocommit, the transaction is now committed (as required by the JDBC API).
 </li><li>The shell script <code>h2.sh</code> did not work with spaces in the path.
     It also works now with quoted spaces in the argument list.
     Thanks a lot to Shimizu Fumiyuki for the patch!

h2/src/main/org/h2/expression/Function.java

@@ -1049,8 +1049,12 @@ public class Function extends Expression implements FunctionCall {
         case SUBSTR:
         case SUBSTRING: {
             String s = v0.getString();
+            int offset = v1.getInt();
+            if (offset < 0) {
+                offset = s.length() + offset + 1;
+            }
             int length = v2 == null ? s.length() : v2.getInt();
-            result = ValueString.get(substring(s, v1.getInt(), length));
+            result = ValueString.get(substring(s, offset, length));
             break;
         }
         case POSITION:

h2/src/test/org/h2/test/testSimple.in.txt

+select substr('[Hello]', 2, 5);
+> Hello;
+select substr('Hello World', -5);
+> World;
 create table test(id null);
 drop table test;
 select select decode(null, null, 'a');