SQL (Structured Query Language) is a powerful language used for managing and manipulating relational databases. Here are some essential SQL commands that every developer should be familiar with:

1. SELECT: Retrieves data from one or more tables.

    SELECT column1, column2 FROM table_name WHERE condition;
   

2. INSERT: Adds new records to a table.

    INSERT INTO table_name (column1, column2) VALUES (value1, value2);
   

3. UPDATE: Modifies existing records in a table.

    UPDATE table_name SET column1 = value1 WHERE condition;
   

4. DELETE: Removes records from a table.

    DELETE FROM table_name WHERE condition;
   

5. CREATE TABLE: Creates a new table with specified columns and data types.

    CREATE TABLE table_name (
        column1 datatype,
        column2 datatype,
        ...
    );
   

6. ALTER TABLE: Modifies an existing table (e.g., add, modify, or drop columns).

    ALTER TABLE table_name
    ADD column_name datatype;
   
    ALTER TABLE table_name
    MODIFY column_name datatype;
   
    ALTER TABLE table_name
    DROP COLUMN column_name;
   

7. DROP TABLE: Deletes an existing table and its data.

    DROP TABLE table_name;
   

8. CREATE INDEX: Creates an index on one or more columns to improve query performance.

    CREATE INDEX index_name
    ON table_name (column1, column2, ...);
   

9. UNION: Combines the result sets of two or more SELECT statements.

    SELECT column1, column2 FROM table1
    UNION
    SELECT column1, column2 FROM table2;
   

10. JOIN: Retrieves data from multiple tables based on a related column.

    SELECT column1, column2
    FROM table1
    INNER JOIN table2 ON table1.column = table2.column;
    

11. GROUP BY: Groups rows based on the values in specified columns.

    SELECT column1, COUNT(*)
    FROM table_name
    GROUP BY column1;
    

12. HAVING: Filters the results of a GROUP BY query.

    SELECT column1, COUNT(*)
    FROM table_name
    GROUP BY column1
    HAVING COUNT(*) > 1;
    

These are fundamental SQL commands, and mastering them will provide a solid foundation for working with relational databases. Depending on the specific database system you're using (e.g., MySQL, PostgreSQL, SQL Server), there may be some variations in syntax and additional features.