Add SUBSTRING(string FROM start_pos [FOR length])

[artemsnisarenko]

Yet another pull request for ANSI SQL-92 substring.

<character substring function> ::=
              SUBSTRING <left paren> <character value expression> FROM <start position>
                          [ FOR <string length> ] <right paren>

http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt

[coveralls]

Coverage decreased (-0.2%) to 86.828% when pulling e9c4f3f on artemsnisarenko:substring-from into 21e8bc4 on JSQLParser:master.

[coveralls]

Coverage increased (+0.1%) to 87.16% when pulling 0863645 on artemsnisarenko:substring-from into feaeaeb on JSQLParser:master.

[wumpz]

Isn't the a from b syntax used as parameter for other functions as well? Wouldn't it be better to improve the function rule to accept this kind of parameter?

[artemsnisarenko]

In SQL:2003 there are four functions that have a from sub-expression. Besides substring they are overlay, trim and extract. The last one already has a dedicated expression class. I'm not sure about merits of either approach but changing the function rule would be inconsistent with what's already there.

[wumpz]

Oracle has extract as well (#579).