Fixed bugs in psql-query.md file

[DivyanshGarg380]

Description:
This PR adds a complete PostgreSQL SQL script that includes:

• Department Table

  • Create department table with columns: dept_name, building, budget, num_staff, and dep_code.
  • Insert initial unique data for CSE, EEE, and ME.
  • Column alterations (ALTER TABLE) for adding/modifying columns.

• Employee & Instructor Queries

  • Insert data from employee to employee_archive with a salary filter.
  • Update instructor salaries using average and conditional logic.
  • Retrieve instructor data with filtering, ordering, limits, offsets, and aggregate functions.

• Joins and Nested Queries

  • LEFT JOIN between employee and branch.
  • Nested query to get employees with total_sales > 30000.
  • Safe explicit INNER JOIN replacing NATURAL JOIN in complex query.

• Triggers

  • triggertest: Inserts data into table2 on insert in table1.
  • sex_trigger: Conditional trigger for inserting a message into trigger_test based on employee sex.

  Changes

• Fixed duplicate inserts and primary key conflicts.

• Added PostgreSQL-compatible default values.

• Replaced unsafe NATURAL JOIN with explicit INNER JOIN.

• Ensured triggers reference existing tables.