@Override
    public Void visit(SubQueryExpression<?> query, Void context) {
        boolean oldInSubsuery = inSubquery;
        inSubquery = true;
        if (inUnion && !templates.isUnionsWrapped()) {
            serialize(query.getMetadata(), false);
        } else {
            append("(");
            serialize(query.getMetadata(), false);
            append(")");
        }
        inSubquery = oldInSubsuery;
        return null;
    }
Return a list of expressions that can be used to uniquely define the query sources

@param joins
@return identifier columns
querydsl-sql/src/main/java/com/querydsl/sql/SQLSerializer.java