How to get values from an expression

Question

I am using grammar from https://github.com/antlr/grammars-v4 and trying to populate column name and value that comes in the where clause. The problem I am having is how to determine if SQLiteParser.Column_nameContext and value context are part of where. It gets little more complex when there is "and" involved.

So what I want is to get "a"/"b" (column) and "g"/"h" (value) in below sql:

select * from table where a = "g" and b = "h";

Answer 1

You seem to be interested in the expr production. In that case, you can attach a base listener to the parse tree and override its enterExpr method:

String sql = "select * from table where a = \"g\" and b = \"h\"";

SQLiteLexer lexer = new SQLiteLexer(new ANTLRInputStream(sql));
SQLiteParser parser = new SQLiteParser(new CommonTokenStream(lexer));
ParseTree tree = parser.select_stmt();

ParseTreeWalker.DEFAULT.walk(new SQLiteBaseListener(){
    @Override
    public void enterExpr(@NotNull SQLiteParser.ExprContext ctx) {
        if (ctx.children.size() == 3 && (ctx.children.get(1) instanceof TerminalNode)) {
            String leftHandSide = ctx.children.get(0).getText();
            String operator = ctx.children.get(1).getText();
            String rightHandSide = ctx.children.get(2).getText();
            System.out.printf("leftHandSide=%s, operator=%s, leftHandSide=%s\n",
                    leftHandSide, operator, rightHandSide);
        }
    }
}, tree);

which will print the following:

leftHandSide=a="g", operator=and, leftHandSide=b="h"
leftHandSide=a, operator==, leftHandSide="g"
leftHandSide=b, operator==, leftHandSide="h"

Instead of performing such an ugly (ctx.children.size() == 3 && (ctx.children.get(1) instanceof TerminalNode)), you can (greatly) improve things by adding labels to the grammar:

ANTLR 4 tree inject/rewrite operator

Also note that the operator precedence in the grammar is wrong, see the following pull request:

https://github.com/antlr/grammars-v4/pull/47

The grammar with fixed operator precedence can also be found here:

https://github.com/bkiers/sqlite-parser/blob/master/src/main/antlr4/nl/bigo/sqliteparser/SQLite.g4