Some examples with Microsoft.SqlServer.TransactSql.ScriptDom and TSql generation and parsing

TransactSqlScriptDomTest.cs

using Microsoft.SqlServer.TransactSql.ScriptDom;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TransactSqlScriptDomTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var sql = @"select firstname, lastname FROM persons as p;
                SELECT id, name FROM companies;
                select s.test from (select 'hello' as test) as s;";

            TSqlParser parser = new TSql120Parser(true);
            IList<ParseError> parseErrors;
            TSqlFragment sqlFragment = parser.Parse(new StringReader(sql), out parseErrors);

            if (parseErrors.Count > 0) Console.WriteLine("Errors:");
            parseErrors.Select(e => e.Message.Indent(2)).ToList().ForEach(Console.WriteLine);

            OwnVisitor visitor = new OwnVisitor();
            sqlFragment.Accept(visitor);

            Console.WriteLine("Done.");
            Console.ReadKey();
        }
    }

    class OwnVisitor : TSqlFragmentVisitor
    {
        public override void ExplicitVisit(SelectStatement node)
        {
            QuerySpecification querySpecification = node.QueryExpression as QuerySpecification;

            FromClause fromClause = querySpecification.FromClause;
            // There could be more than one TableReference!
            // TableReference is not sure to be a NamedTableReference, could be as example a QueryDerivedTable
            NamedTableReference namedTableReference = fromClause.TableReferences[0] as NamedTableReference;
            TableReferenceWithAlias tableReferenceWithAlias = fromClause.TableReferences[0] as TableReferenceWithAlias;
            string baseIdentifier = namedTableReference?.SchemaObject.BaseIdentifier?.Value;
            string schemaIdentifier = namedTableReference?.SchemaObject.SchemaIdentifier?.Value;
            string databaseIdentifier = namedTableReference?.SchemaObject.DatabaseIdentifier?.Value;
            string serverIdentifier = namedTableReference?.SchemaObject.ServerIdentifier?.Value;
            string alias = tableReferenceWithAlias.Alias?.Value;
            Console.WriteLine("From:");
            Console.WriteLine($"  {"Server:",-10} {serverIdentifier}");
            Console.WriteLine($"  {"Database:",-10} {databaseIdentifier}");
            Console.WriteLine($"  {"Schema:",-10} {schemaIdentifier}");
            Console.WriteLine($"  {"Table:",-10} {baseIdentifier}");
            Console.WriteLine($"  {"Alias:",-10} {alias}");



            // Example of changing the alias:
            //(fromClause.TableReferences[0] as NamedTableReference).Alias = new Identifier() { Value = baseIdentifier[0].ToString() };

            Console.WriteLine("Statement:");
            Console.WriteLine(node.ToSqlString().Indent(2));

            Console.WriteLine("¯".Multiply(40));

            base.ExplicitVisit(node);
        }
    }

    public static class TSqlDomHelpers
    {
        public static string ToSourceSqlString(this TSqlFragment fragment)
        {
            StringBuilder sqlText = new StringBuilder();
            for (int i = fragment.FirstTokenIndex; i <= fragment.LastTokenIndex; i++)
            {
                sqlText.Append(fragment.ScriptTokenStream[i].Text);
            }
            return sqlText.ToString();
        }

        public static string ToSqlString(this TSqlFragment fragment)
        {
            SqlScriptGenerator generator = new Sql120ScriptGenerator();
            string sql;
            generator.GenerateScript(fragment, out sql);
            return sql;
        }
    }
    public static class StringHelpers
    {
        public static string Indent(this string Source, int NumberOfSpaces)
        {
            string indent = new string (' ', NumberOfSpaces);
            return indent + Source.Replace("\n", "\n" + indent);
        }
        public static string Multiply(this string Source, int Multiplier)
        {
            StringBuilder stringBuilder = new StringBuilder(Multiplier * Source.Length);
            for (int i = 0; i < Multiplier; i++)
            {
                stringBuilder.Append(Source);
            }
            return stringBuilder.ToString();
        }
    }
}