Merge subqueries chapter into Restriction and rename Query Operators

- Combined subquery patterns into the Restriction chapter since
  subqueries are just a special case of restrictions (semijoins/antijoins)
- Added Pattern 6 (Universal Quantification), Pattern 7 (Reverse Perspective),
  Self-Referencing Patterns, and Building Queries Systematically sections
- Added Summary of Patterns table for quick reference
- Added more practice exercises covering advanced subquery patterns
- Renamed "The Query Operators" to "Query Operators"
- Deleted the now-redundant 080-subqueries.ipynb chapter

Author: claude

book/50-queries/010-operators.ipynb

@@ -3,19 +3,7 @@
   {
    "cell_type": "markdown",
    "metadata": {},
-   "source": [
-    "# The Query Operators\n",
-    "\n",
-    "DataJoint provides five operators for building queries. These operators form a complete query language—any question you can ask of your data can be expressed using these five tools.\n",
-    "\n",
-    "| Operator | Symbol | Purpose |\n",
-    "|----------|--------|--------|\n",
-    "| **Restriction** | `&`, `-` | Filter entities by conditions |\n",
-    "| **Projection** | `.proj()` | Select and compute attributes |\n",
-    "| **Join** | `*` | Combine related entities |\n",
-    "| **Aggregation** | `.aggr()` | Summarize related data |\n",
-    "| **Union** | `+` | Combine entity sets of the same type |"
-   ]
+   "source": "# Query Operators\n\nDataJoint provides five operators for building queries. These operators form a complete query language—any question you can ask of your data can be expressed using these five tools.\n\n| Operator | Symbol | Purpose |\n|----------|--------|--------|\n| **Restriction** | `&`, `-` | Filter entities by conditions |\n| **Projection** | `.proj()` | Select and compute attributes |\n| **Join** | `*` | Combine related entities |\n| **Aggregation** | `.aggr()` | Summarize related data |\n| **Union** | `+` | Combine entity sets of the same type |"
   },
   {
    "cell_type": "markdown",
@@ -259,4 +247,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}