feat: SQL-like query and column statistics

CsvSharp/Program.cs

@@ -61,6 +61,16 @@
         PrintTable(headers, records.Take(limit));
         break;
 
+    case "query":
+        var sql = args.ElementAtOrDefault(2);
+        if (sql == null) { Console.Error.WriteLine("Usage: csvsharp query <file.csv> '<SQL>'"); return 1; }
+        QueryTable(headers, records, sql);
+        break;
+
+    case "stats":
+        ShowStats(headers, records);
+        break;
+
     default:
         Console.Error.WriteLine($"Unknown command: {command}");
         return 1;
@@ -95,7 +105,6 @@
     var colWidths = headers.ToDictionary(h => h, h => Math.Max(h.Length,
         rows.Any() ? rows.Max(r => r.GetValueOrDefault(h, "").Length) : 0));
 
-    // Header
     Console.WriteLine(string.Join(" | ", headers.Select(h => h.PadRight(colWidths[h]))));
     Console.WriteLine(string.Join("-|-", headers.Select(h => new string('-', colWidths[h]))));
 
@@ -105,3 +114,70 @@
             headers.Select(h => (row.GetValueOrDefault(h, "")).PadRight(colWidths[h]))));
     }
 }
+
+static void QueryTable(string[] headers, List<Dictionary<string, string>> records, string sql)
+{
+    // Simple SQL-like: "SELECT col1, col2 WHERE col3 = 'value' ORDER BY col1 LIMIT 10"
+    var selectMatch = Regex.Match(sql, @"SELECT\s+(.+?)(?:\s+WHERE\s+(.+?))?(?:\s+ORDER\s+BY\s+(\w+)(?:\s+(ASC|DESC))?)?(?:\s+LIMIT\s+(\d+))?", RegexOptions.IgnoreCase);
+    if (!selectMatch.Success) { Console.Error.WriteLine("Invalid query syntax."); return; }
+
+    var selectCols = selectMatch.Groups[1].Value == "*"
+        ? headers.ToList()
+        : selectMatch.Groups[1].Value.Split(',').Select(c => c.Trim()).Where(h => headers.Contains(h)).ToList();
+
+    var whereClause = selectMatch.Groups[2].Value;
+    var orderBy = selectMatch.Groups[3].Value;
+    var orderDir = selectMatch.Groups[4].Success ? selectMatch.Groups[4].Value : "ASC";
+    var limit = selectMatch.Groups[5].Success ? int.Parse(selectMatch.Groups[5].Value) : int.MaxValue;
+
+    var filtered = records.AsEnumerable();
+    if (!string.IsNullOrEmpty(whereClause))
+    {
+        var whereMatch = Regex.Match(whereClause, @"(\w+)\s*(=|!=|>|<|>=|<=)\s*'?(.+?)'?$");
+        if (whereMatch.Success)
+        {
+            var col = whereMatch.Groups[1].Value;
+            var op = whereMatch.Groups[2].Value;
+            var val = whereMatch.Groups[3].Value.Trim('\'');
+            filtered = filtered.Where(r =>
+            {
+                if (!r.ContainsKey(col)) return false;
+                var rv = r[col];
+                return op switch
+                {
+                    "=" => rv == val,
+                    "!=" => rv != val,
+                    ">" => string.Compare(rv, val, StringComparison.Ordinal) > 0,
+                    "<" => string.Compare(rv, val, StringComparison.Ordinal) < 0,
+                    _ => false
+                };
+            });
+        }
+    }
+
+    if (!string.IsNullOrEmpty(orderBy) && headers.Contains(orderBy))
+    {
+        filtered = orderDir == "DESC"
+            ? filtered.OrderByDescending(r => r.GetValueOrDefault(orderBy, ""))
+            : filtered.OrderBy(r => r.GetValueOrDefault(orderBy, ""));
+    }
+
+    var result = filtered.Take(limit).ToList();
+    PrintTable(selectCols.ToArray(), result);
+    Console.WriteLine($"\n{result.Count} row(s)");
+}
+
+static void ShowStats(string[] headers, List<Dictionary<string, string>> records)
+{
+    Console.WriteLine($"Rows: {records.Count}");
+    Console.WriteLine($"Columns: {headers.Length}");
+    Console.WriteLine($"\nColumn stats:");
+    foreach (var h in headers)
+    {
+        var values = records.Select(r => r.GetValueOrDefault(h, "")).Where(v => !string.IsNullOrEmpty(v)).ToList();
+        var nums = values.Select(v => { double.TryParse(v, out var n); return (ok: double.TryParse(v, out n), val: n); })
+                        .Where(x => x.ok).Select(x => x.val).ToList();
+        Console.WriteLine($"  {h}: {values.Count} non-empty" +
+            (nums.Any() ? $", min={nums.Min():F2}, max={nums.Max():F2}, avg={nums.Average():F2}" : ""));
+    }
+}