Scala Parser Combinators - SQL Parser Example

Scala Parser Combinators: https://github.com/scala/scala-parser-combinators

Scala Parser Combinators is basically a parsing framework for extracting data when there is a pattern in the given input. This framework provides a more statically typed, functional way of extracting instead of using regex expression which can get hard to read.

In this post, lets build a SQL parser where given a valid sql statement we can identify the "table" name, "column" names and other sql properties. Following are some fundamental functions, operations that Scala combinator provides which would help in parsing:

• " | ": says “succeed if either the left or right operand parse successfully”
• " ~ ": says “succeed if the left operand parses successfully, and then the right parses successfully on the remaining input”
• " ~> ": says “succeed if the left operand parses successfully followed by the right, but do not include the left content in the result”
• " <~ ": is the reverse, “succeed if the left operand is parsed successfully followed by the right, but do not include the right content in the result”
• " ^^ ": says “if the left operand parses successfully, transform the result using the function on the right”
• " ^^^ ": says “if the left operand parses successfully, ignore the result and use the value from the right”
• " rep(fn) ": says "parse the given input using the parser function fn"
• " repsep(ident, char) ": says "parse the given input and split the input using the given 'char'"

Lets start out with a set of SQL statements and it's associated Parser code

select * from users

select name,age from users

select count(name) from users

select * from users order by age desc

select * from users order by name, age desc

select age from users where age>30