docs(notes): add IBM DB2 advanced and JCC documentation

Author: lzwjava

notes/2025-03-23-ibm-db2-advanced-en.md

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+---
+title: IBM DB2 SQL Advanced Operations
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+I'll teach you some advanced SQL operations in IBM DB2. Let's dive into nested queries, complex conditions, and advanced grouping techniques.
+
+## Nested Queries (Subqueries)
+
+Subqueries are queries embedded within other queries, often used in WHERE, HAVING, or FROM clauses.
+
+### In WHERE Clause:
+```sql
+SELECT employee_id, last_name, salary
+FROM employees
+WHERE salary > (SELECT AVG(salary) FROM employees);
+```
+This returns employees with salary above the average.
+
+### Correlated Subquery:
+```sql
+SELECT d.dept_name, e.last_name
+FROM departments d
+WHERE EXISTS (
+    SELECT 1 
+    FROM employees e 
+    WHERE e.dept_id = d.dept_id 
+    AND e.salary > 100000
+);
+```
+This finds departments that have at least one employee earning over $100,000.
+
+### In FROM Clause (Derived Tables):
+```sql
+SELECT dept_name, avg_salary
+FROM (
+    SELECT d.dept_name, AVG(e.salary) as avg_salary
+    FROM departments d
+    JOIN employees e ON d.dept_id = e.dept_id
+    GROUP BY d.dept_name
+) AS dept_stats
+WHERE avg_salary > 75000;
+```
+
+## Complex WHERE Conditions
+
+### Using CASE in WHERE:
+```sql
+SELECT product_id, product_name, price
+FROM products
+WHERE 
+    CASE 
+        WHEN category = 'Electronics' THEN price < 1000
+        WHEN category = 'Furniture' THEN price < 500
+        ELSE price < 200
+    END;
+```
+
+### Multiple Conditions with Combinations:
+```sql
+SELECT *
+FROM orders
+WHERE 
+    (status = 'Pending' AND created_date > CURRENT DATE - 7 DAYS)
+    OR
+    (status = 'Processing' AND created_date > CURRENT DATE - 14 DAYS)
+    OR
+    (customer_id IN (SELECT customer_id FROM premium_customers));
+```
+
+### Using BETWEEN and IN:
+```sql
+SELECT order_id, customer_id, total_amount
+FROM orders
+WHERE 
+    order_date BETWEEN '2024-01-01' AND '2024-03-31'
+    AND
+    shipping_method IN ('Express', 'Priority')
+    AND
+    total_amount > 1000;
+```
+
+## Advanced GROUP BY Operations
+
+### GROUP BY with ROLLUP:
+```sql
+SELECT 
+    COALESCE(region, 'All Regions') as region,
+    COALESCE(country, 'All Countries') as country,
+    SUM(sales) as total_sales
+FROM sales_data
+GROUP BY ROLLUP(region, country);
+```
+This produces subtotals for each group level and a grand total.
+
+### GROUP BY with CUBE:
+```sql
+SELECT 
+    COALESCE(year, 0) as year,
+    COALESCE(quarter, 0) as quarter,
+    COALESCE(product_line, 'All Products') as product_line,
+    SUM(revenue) as total_revenue
+FROM sales_data
+GROUP BY CUBE(year, quarter, product_line);
+```
+This generates all possible combinations of the specified dimensions.
+
+### GROUPING SETS:
+```sql
+SELECT 
+    product_category,
+    region,
+    SUM(sales) as total_sales
+FROM sales_data
+GROUP BY GROUPING SETS(
+    (product_category, region),
+    (product_category),
+    (region),
+    ()
+);
+```
+This specifies multiple grouping combinations in a single query.
+
+## Common Table Expressions (CTEs)
+
+```sql
+WITH regional_sales AS (
+    SELECT region, SUM(amount) as total_sales
+    FROM orders
+    GROUP BY region
+),
+top_regions AS (
+    SELECT region
+    FROM regional_sales
+    ORDER BY total_sales DESC
+    FETCH FIRST 3 ROWS ONLY
+)
+SELECT o.order_id, o.customer_id, o.amount
+FROM orders o
+JOIN top_regions tr ON o.region = tr.region
+WHERE o.amount > 1000;
+```
+
+## Advanced JOINS
+
+### Self JOIN:
+```sql
+SELECT e.employee_id, e.last_name as employee, m.last_name as manager
+FROM employees e
+LEFT JOIN employees m ON e.manager_id = m.employee_id;
+```
+
+### Multiple JOINS with Complex Conditions:
+```sql
+SELECT 
+    c.customer_name,
+    p.product_name,
+    o.order_date,
+    SUM(od.quantity * od.unit_price) as line_total
+FROM customers c
+JOIN orders o ON c.customer_id = o.customer_id
+JOIN order_details od ON o.order_id = od.order_id
+JOIN products p ON od.product_id = p.product_id
+WHERE 
+    o.order_date > '2024-01-01'
+    AND p.category IN ('Electronics', 'Appliances')
+    AND c.country = 'USA'
+GROUP BY c.customer_name, p.product_name, o.order_date
+HAVING SUM(od.quantity * od.unit_price) > 500
+ORDER BY line_total DESC;
+```
+
+## Window Functions
+
+```sql
+SELECT 
+    dept_id,
+    employee_id,
+    salary,
+    AVG(salary) OVER (PARTITION BY dept_id) as dept_avg_salary,
+    salary - AVG(salary) OVER (PARTITION BY dept_id) as diff_from_avg,
+    RANK() OVER (PARTITION BY dept_id ORDER BY salary DESC) as salary_rank
+FROM employees;
+```
+
+Would you like me to elaborate on any of these techniques or provide more specific examples for a particular DB2 scenario?