#253 Support for subqueries

Author: mk3008

src/QueryBuilderByLinq/SelectQueryBuilder.cs

@@ -108,7 +108,14 @@ private SelectQuery BuildAsRoot(MethodCallExpression expression)
 		{
 			if (from.Value is IQueryable q && q.Provider is TableQuery tq)
 			{
-				sq.From(tq.TableName).As(fromAlias.Name!);
+				if (tq.InnerQuery != null)
+				{
+					sq.From(tq.InnerQuery.ToQueryAsPostgres()).As(fromAlias.Name!);
+				}
+				else
+				{
+					sq.From(tq.TableName).As(fromAlias.Name!);
+				}
 			}
 			else
 			{

src/QueryBuilderByLinq/Sql.cs

@@ -26,6 +26,11 @@ public static IQueryable<T> From<T>(string tableName)
 		return new Table<T>(tableName);
 	}
 
+	public static IQueryable<T> From<T>(IQueryable<T> subquery)
+	{
+		return new Table<T>(subquery);
+	}
+
 	public static IQueryable<T> InnerJoin<T>(Expression<Predicate<T>> condition)
 	{
 		return new Table<T>();

src/QueryBuilderByLinq/Table.cs

@@ -1,4 +1,5 @@
-using System.Collections;
+using Carbunql.Clauses;
+using System.Collections;
 using System.Linq.Expressions;
 
 namespace QueryBuilderByLinq;
@@ -15,6 +16,11 @@ public Table(string tableName)
 		Query = new TableQuery<T>() { TableName = tableName };
 	}
 
+	public Table(IQueryable<T> subquery)
+	{
+		Query = new TableQuery<T>() { InnerQuery = subquery.AsQueryable() };
+	}
+
 	private IQueryable<T> Query { get; set; }
 
 	public Type ElementType => Query.ElementType;

src/QueryBuilderByLinq/TableQuery.cs

@@ -5,7 +5,9 @@ namespace QueryBuilderByLinq;
 
 public abstract class TableQuery
 {
-	public string TableName { get; set; } = string.Empty;
+	public string TableName { get; internal set; } = string.Empty;
+
+	public IQueryable? InnerQuery { get; internal set; }
 }
 
 public class TableQuery<T> : TableQuery, IOrderedQueryable<T>, IQueryProvider

test/QueryBuilderByLinq.Test/SubQueryTest.cs

@@ -0,0 +1,47 @@
+using Carbunql;
+using Xunit.Abstractions;
+using static QueryBuilderByLinq.Sql;
+
+namespace QueryBuilderByLinq.Test;
+
+public class SubQueryTest
+{
+	private readonly QueryCommandMonitor Monitor;
+
+	public SubQueryTest(ITestOutputHelper output)
+	{
+		Monitor = new QueryCommandMonitor(output);
+		Output = output;
+	}
+
+	private ITestOutputHelper Output { get; set; }
+
+	[Fact]
+	public void DefaultTest()
+	{
+		var subq = from a in From<table_a>() select new { ID = a.a_id, Text = a.text };
+		var query = from x in From(subq) select new { x.ID, x.Text };
+
+		var sq = query.ToQueryAsPostgres();
+
+		Monitor.Log(sq);
+
+		var sql = @"
+SELECT
+    x.ID,
+    x.Text
+FROM
+    (
+        SELECT
+            a.a_id AS ID,
+            a.text AS Text
+        FROM
+            table_a AS a
+    ) AS x";
+
+		Assert.Equal(29, sq.GetTokens().ToList().Count);
+		Assert.Equal(sql.ToValidateText(), sq.ToText().ToValidateText());
+	}
+
+	public record struct table_a(int a_id, string text, int value);
+}