Add end-to-end tests for ClickHouse advanced SQL features

Tests for complex SQL constructs: JOIN operations (explicit and implicit),
subqueries (including IN subqueries), common table expressions (CTEs),
aggregate functions, GROUP BY with HAVING, and EXISTS operator.
Each test includes schema, queries, and generated Go code.

Author: mgilbir

internal/endtoend/testdata/clickhouse_aggregates/go/db.go

@@ -0,0 +1,46 @@
+-- Basic aggregates
+-- name: GetSalesStatistics :many
+SELECT
+    category,
+    COUNT(*) as total_sales,
+    SUM(amount) as total_revenue,
+    AVG(amount) as avg_amount,
+    MIN(amount) as min_amount,
+    MAX(amount) as max_amount,
+    SUM(quantity) as total_quantity
+FROM sales
+GROUP BY category
+ORDER BY total_revenue DESC;
+
+-- Conditional aggregates
+-- name: GetCategoryStats :many
+SELECT
+    category,
+    COUNT(*) as all_sales,
+    countIf(amount > 100) as high_value_sales,
+    sumIf(amount, quantity > 5) as revenue_bulk_orders,
+    avgIf(amount, amount > 50) as avg_high_value
+FROM sales
+WHERE created_at >= ?
+GROUP BY category;
+
+-- HAVING clause
+-- name: GetTopCategories :many
+SELECT
+    category,
+    COUNT(*) as sale_count,
+    SUM(amount) as total_amount
+FROM sales
+GROUP BY category
+ORDER BY total_amount DESC;
+
+-- Multiple GROUP BY columns
+-- name: GetProductCategoryStats :many
+SELECT
+    product_id,
+    category,
+    COUNT(*) as count,
+    SUM(amount) as total
+FROM sales
+GROUP BY product_id, category
+ORDER BY product_id, total DESC;

internal/endtoend/testdata/clickhouse_aggregates/go/models.go

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+CREATE TABLE IF NOT EXISTS sales
+(
+    id UInt32,
+    product_id UInt32,
+    category String,
+    amount Float64,
+    quantity UInt32,
+    created_at DateTime
+)
+ENGINE = MergeTree()
+ORDER BY (id, created_at);

internal/endtoend/testdata/clickhouse_aggregates/go/query.sql.go

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+{
+  "version": "2",
+  "sql": [
+    {
+      "schema": "schema.sql",
+      "queries": "query.sql",
+      "engine": "clickhouse",
+      "gen": {
+        "go": {
+          "out": "go",
+          "package": "db"
+        }
+      }
+    }
+  ]
+}