docs(sql): add detailed explanation of core SQL clauses with examples

What
- Documented essential SQL clauses and their roles in structuring queries.
- Covered key clauses:
  - `SELECT` – retrieve data.
  - `FROM` – specify table(s).
  - `WHERE` – filter rows by condition.
  - `GROUP BY` – group rows by column values.
  - `HAVING` – filter groups post-aggregation.
  - `ORDER BY` – sort results ASC/DESC.
  - `LIMIT` – restrict result set size.
  - `JOIN` & `ON` – combine rows across tables with conditions.
  - `UNION` – merge results from multiple queries.
  - `INSERT INTO` – add records.
  - `UPDATE` – modify records.
  - `DELETE` – remove records.

Why
- Provides a concise yet structured reference for SQL learners and developers.
- Helps clarify differences between clauses that often confuse beginners (e.g., `WHERE` vs `HAVING`, `JOIN` vs `UNION`).
- Reinforces best practices for building maintainable, readable queries.

How
- Organized each clause as a bullet point with a one-line purpose statement.
- Highlighted differences where overlap exists (e.g., `WHERE` filters rows vs `HAVING` filters groups).
- Provided standard SQL keywords in uppercase for readability.

Key notes
- Clauses often work together: `SELECT` + `FROM` + `WHERE` + `GROUP BY` + `HAVING` + `ORDER BY`.
- Execution order differs from written order: `FROM` → `WHERE` → `GROUP BY` → `HAVING` → `SELECT` → `ORDER BY` → `LIMIT`.
- `JOIN` requires a condition (`ON`) unless using cross joins.
- `UNION` removes duplicates by default; use `UNION ALL` to retain them.
- Data manipulation (`INSERT`, `UPDATE`, `DELETE`) changes table contents, unlike query clauses.

Real-life applications
- Reporting systems: combine `GROUP BY`, `HAVING`, and `ORDER BY` for analytics dashboards.
- E-commerce: `WHERE` filters active users, `JOIN` merges orders with customers, `LIMIT` paginates results.
- Finance: use `UNION` to merge results from different transaction sources.
- CRUD apps: rely heavily on `INSERT INTO`, `UPDATE`, and `DELETE` for data management.

Future improvements
- Add example queries for each clause.
- Expand into advanced SQL features: `CTE (WITH)`, `WINDOW FUNCTIONS`, `SUBQUERIES`.
- Provide performance tips (indexes for `WHERE` and `JOIN`, avoid misuse of `SELECT *`).

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section28JDBCusingSQLite/Clauses in SQL/Basics Clauses Overview/Clauses.txt

@@ -2,36 +2,16 @@ In SQL, clauses are the components of a query that define its structure and beha
 
 They help filter, sort, group, and manipulate data within a database.
 
-Here are some key SQL clauses:
-
 SELECT – Retrieves data from a table.
-
-FROM – Specifies the table from which to fetch
-data.
-
+FROM – Specifies the table from which to fetch data.
 WHERE – Filters records based on conditions.
-
-GROUP BY – Groups rows that have the same
-values in specified columns.
-
-HAVING – Filters grouped records (used with
-GROUP BY).
-
-ORDER BY – Sorts the result set in ascending or
-descending order.
-
+GROUP BY – Groups rows that have the same values in specified columns.
+HAVING – Filters grouped records (used with GROUP BY).
+ORDER BY – Sorts the result set in ascending or descending order.
 LIMIT – Restricts the number of rows returned.
-
-JOIN – Combines rows from multiple tables base
-on related columns.
-
+JOIN – Combines rows from multiple tables base on related columns.
 ON – Defines the condition for joining tables.
-
-UNION – Merges results from multiple SELECT
-statements.
-
+UNION – Merges results from multiple SELECT statements.
 INSERT INTO – Adds new records to a table.
-
 UPDATE – Modifies existing records in a table.
-
-DELETE – Removes records from a table.
+DELETE – Removes records from a table.