[jOOQ/jOOQ#10523] Fix this for dialects that do not support correlated derived tables

jOOQ/src/main/java/org/jooq/impl/FieldMapForUpdate.java

@@ -258,14 +258,12 @@ final class FieldMapForUpdate extends AbstractQueryPartMap<FieldOrRow, FieldOrRo
                     visitSubquery(ctx, select, false, false, false);
                 }
                 else {
-                    Field<?>[] f = Tools.fields(size);
-
                     for (int i = 0; i < size; i++) {
                         FieldMapForUpdate mu = new FieldMapForUpdate(table, setClause, null);
                         separator = mu.acceptAssignmentClause(ctx,
                             supportsQualify,
                             row.field(i),
-                            field(select(f[i]).from(select.asTable(table(name("t")), f))),
+                            new ProjectSingleScalarSubquery<>(select, i),
                             separator
                         );
                     }

jOOQ/src/main/java/org/jooq/impl/ProjectSingleScalarSubquery.java

@@ -0,0 +1,86 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Other licenses:
+ * -----------------------------------------------------------------------------
+ * Commercial licenses for this work are available. These replace the above
+ * ASL 2.0 and offer limited warranties, support, maintenance, and commercial
+ * database integrations.
+ *
+ * For more information, please visit: http://www.jooq.org/licenses
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ */
+
+package org.jooq.impl;
+
+import static org.jooq.impl.DSL.name;
+import static org.jooq.impl.DSL.select;
+import static org.jooq.impl.DSL.table;
+import static org.jooq.impl.Names.N_SELECT;
+
+import java.util.Arrays;
+
+import org.jooq.Context;
+import org.jooq.DataType;
+import org.jooq.Field;
+import org.jooq.Record1;
+import org.jooq.Select;
+
+/**
+ * A scalar subquery that wraps a non-scalar subquery, but projects only one of
+ * its columns either by SQL transformation, or via a derived table.
+ *
+ * @author Lukas Eder
+ */
+@SuppressWarnings("unchecked")
+final class ProjectSingleScalarSubquery<T> extends AbstractField<T> implements QOM.UTransient {
+
+    final Select<?> query;
+    final int       index;
+
+    ProjectSingleScalarSubquery(Select<?> query, int index) {
+        super(N_SELECT, (DataType<T>) query.getSelect().get(index).getDataType());
+
+        this.query = query;
+        this.index = index;
+    }
+
+    @Override
+    public final void accept(Context<?> ctx) {
+        SelectQueryImpl<?> q = Tools.selectQueryImpl(query);
+
+        if (q.$distinct()
+            || !q.$orderBy().isEmpty()
+            || q.hasUnions()) {
+            Field<?>[] f = Tools.fields(query.getSelect().size());
+            ctx.visit(DSL.field(select(f[index]).from(query.asTable(table(name("t")), f))));
+        }
+        else
+            ctx.visit(DSL.field((Select<Record1<T>>) query.$select(Arrays.asList(query.$select().get(index)))));
+    }
+}