SQL Syntax, Keywords & DBMS vs RDBMS

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-- SQL is case-INSENSITIVE for keywords:
-- SELECT = select = Select = sElEcT (all equivalent)
-- But data values ARE case-sensitive:
-- WHERE Name = 'Ravi'  ≠  WHERE Name = 'ravi'

-- Basic statement structure:
SELECT column1, column2     -- Columns to retrieve
FROM   table_name           -- Source table
WHERE  condition            -- Filter rows (optional)
ORDER BY column1 ASC;       -- Sort result (optional)
-- Every SQL statement ends with semicolon (;)

-- Identifier quoting (for reserved words or special chars):
SELECT "select", "my-column", "Table Name"   -- PostgreSQL/standard
FROM   [my table]                            -- SQL Server
FROM   `my table`;                           -- MySQL

-- String literals use single quotes (ANSI standard):
WHERE Name = 'Ravi Kumar'    -- Correct
WHERE Name = "Ravi Kumar"    -- Non-standard (works in MySQL, not Oracle/PostgreSQL)

-- NULL is not a value — use IS NULL, not = NULL:
WHERE DeptID IS NULL      -- Correct
WHERE DeptID = NULL       -- ALWAYS FALSE (NULL != NULL in SQL)

-- Aliases
SELECT Name AS FullName, Salary * 12 AS AnnualSalary FROM Employee;
SELECT Name FullName FROM Employee;   -- AS is optional

-- Written order:      Logical execution order:
-- 1. SELECT           5. SELECT (choose columns, compute expressions)
-- 2. FROM             1. FROM (identify source tables)
-- 3. JOIN             1. JOIN (combine tables)
-- 4. WHERE            2. WHERE (filter rows)
-- 5. GROUP BY         3. GROUP BY (form groups)
-- 6. HAVING           4. HAVING (filter groups)
-- 7. ORDER BY         6. ORDER BY (sort)
-- 8. LIMIT            7. LIMIT/OFFSET (paginate)

-- Full example showing all clauses:
SELECT DISTINCT           -- Remove duplicate output rows
    d.DeptName,
    COUNT(e.EmpID)   AS HeadCount,
    AVG(e.Salary)    AS AvgSalary,
    MAX(e.Salary)    AS TopSalary
FROM Employee e
INNER JOIN Department d ON e.DeptID = d.DeptID
WHERE e.Status = 'Active'
  AND e.JoinDate > '2020-01-01'
GROUP BY d.DeptName
HAVING COUNT(e.EmpID) > 5
ORDER BY AvgSalary DESC
LIMIT 10 OFFSET 0;        -- First page of 10 results

-- DISTINCT vs GROUP BY:
SELECT DISTINCT DeptID FROM Employee;      -- Unique dept values
SELECT DeptID, COUNT(*) FROM Employee GROUP BY DeptID;  -- Unique + counts