Какие комментарии SQL являются действительными?

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# Hash comment (MySQL only)

┌──────────────────────────────────────────────────────────────────┐
│            LOGICAL EXECUTION ORDER OF SELECT                     │
├────────┬─────────────────────────────────────────────────────────┤
│ 1      │ FROM + JOINs → Determine source tables                  │
│ 2      │ WHERE → Filter rows                                     │
│ 3      │ GROUP BY → Aggregate into buckets                       │
│ 4      │ HAVING → Filter groups                                  │
│ 5      │ SELECT → Compute columns & aliases                      │
│ 6      │ DISTINCT → Remove duplicates                            │
│ 7      │ ORDER BY → Sort results                                 │
│ 8      │ LIMIT / OFFSET → Paginate                               │
└────────┴─────────────────────────────────────────────────────────┘

-- ❌ ERROR: alias "full_name" doesn't exist yet at step 2 (WHERE)
SELECT first_name || ' ' || last_name AS full_name
FROM employees
WHERE full_name = 'John Doe';

-- ✅ FIX: Repeat the expression or use a subquery/CTE
SELECT first_name || ' ' || last_name AS full_name
FROM employees
WHERE first_name || ' ' || last_name = 'John Doe';

-- Comparison operators
WHERE salary > 50000
WHERE age BETWEEN 18 AND 65          -- Inclusive on both ends
WHERE department IN ('Engineering', 'Product', 'Design')
WHERE name LIKE 'A%'                  -- Starts with A
WHERE name LIKE '%son'                -- Ends with son
WHERE name LIKE '_a%'                 -- Second letter is 'a'
WHERE name LIKE '%\_%' ESCAPE '\'     -- Contains literal underscore
WHERE email IS NULL                   -- Missing email (NOT = NULL!)
WHERE email IS NOT NULL

-- 🚨 WRONG: This returns NOTHING, not even rows where manager_id IS NULL
SELECT * FROM employees WHERE manager_id = NULL;

-- ✅ CORRECT:
SELECT * FROM employees WHERE manager_id IS NULL;

-- COALESCE gives you a fallback value:
SELECT name, COALESCE(phone, 'No phone') AS contact FROM users;
-- IFNULL is MySQL-specific; COALESCE is standard and accepts multiple args:
SELECT COALESCE(mobile, home_phone, work_phone, 'No phone') FROM contacts;

-- Multi-column sort
SELECT * FROM products
ORDER BY category ASC, price DESC;

-- Pagination (Page 3, 20 items per page)
-- MySQL / PostgreSQL
SELECT * FROM products ORDER BY id LIMIT 20 OFFSET 40;

-- SQL Server
SELECT * FROM products ORDER BY id OFFSET 40 ROWS FETCH NEXT 20 ROWS ONLY;

-- Offset gets slower as page number grows!
-- For large tables, use keyset pagination:
SELECT * FROM products WHERE id > 1000 ORDER BY id LIMIT 20;

-- Find departments with average salary > 60k, sorted
SELECT 
    department,
    COUNT(*) AS headcount,
    AVG(salary) AS avg_salary,
    MAX(salary) AS top_salary
FROM employees
WHERE hire_date >= '2020-01-01'    -- Filter ROWS first
GROUP BY department
HAVING AVG(salary) > 60000         -- Filter GROUPS after aggregation
ORDER BY avg_salary DESC;

┌─────────────────────────────────────────────────────────────────┐
│ WHERE filters ROWS (before grouping)                            │
│ HAVING filters GROUPS (after aggregation)                       │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│  Raw Rows → [WHERE] → Filtered Rows → [GROUP BY] → Groups      │
│                                                           ↓     │
│  Result ← [SELECT] ← [HAVING] ← Filtered Groups                 │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘