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DB - Aggregate functions
Signed-off-by: Serhii Horodilov <sgorodil@gmail.com>
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‎assets/img/aggregate-function.svg‎

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‎assets/sql/products.pg13-1.dump.sql‎

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--
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-- PostgreSQL database dump
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--
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-- Dumped from database version 13.1 (Debian 13.1-1.pgdg100+1)
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-- Dumped by pg_dump version 14.5
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SET statement_timeout = 0;
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SET lock_timeout = 0;
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SET idle_in_transaction_session_timeout = 0;
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SET client_encoding = 'UTF8';
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SET standard_conforming_strings = on;
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SELECT pg_catalog.set_config('search_path', '', false);
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SET check_function_bodies = false;
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SET xmloption = content;
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SET client_min_messages = warning;
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SET row_security = off;
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SET default_tablespace = '';
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SET default_table_access_method = heap;
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--
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-- Name: product; Type: TABLE; Schema: public; Owner: postgres
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--
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CREATE TABLE public.product (
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id integer NOT NULL,
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name text NOT NULL,
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price integer NOT NULL,
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category text NOT NULL,
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sold integer NOT NULL
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);
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ALTER TABLE public.product OWNER TO postgres;
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--
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-- Name: product_id_seq; Type: SEQUENCE; Schema: public; Owner: postgres
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--
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CREATE SEQUENCE public.product_id_seq
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AS integer
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START WITH 1
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INCREMENT BY 1
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NO MINVALUE
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NO MAXVALUE
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CACHE 1;
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ALTER TABLE public.product_id_seq OWNER TO postgres;
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--
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-- Name: product_id_seq; Type: SEQUENCE OWNED BY; Schema: public; Owner: postgres
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--
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ALTER SEQUENCE public.product_id_seq OWNED BY public.product.id;
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--
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-- Name: product id; Type: DEFAULT; Schema: public; Owner: postgres
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--
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ALTER TABLE ONLY public.product ALTER COLUMN id SET DEFAULT nextval('public.product_id_seq'::regclass);
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--
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-- Data for Name: product; Type: TABLE DATA; Schema: public; Owner: postgres
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--
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COPY public.product (id, name, price, category, sold) FROM stdin;
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1 Samsung Galaxy 100000 Phones 500
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2 iPhone 13 Pro 120000 Phones 300
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3 MacBook Pro 350000 Laptops 100
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4 Lenovo ThinkPad 150000 Laptops 200
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5 HP Printer 20000 Printers 800
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6 Dell Monitor 50000 Monitors 400
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7 Sony Headphones 30000 Audio 0
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8 Bose Soundbar 70000 Audio 0
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9 Xbox Series X 60000 Gaming 250
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10 PlayStation 5 55000 Gaming 350
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\.
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--
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-- Name: product_id_seq; Type: SEQUENCE SET; Schema: public; Owner: postgres
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--
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SELECT pg_catalog.setval('public.product_id_seq', 10, true);
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--
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-- Name: product product_pkey; Type: CONSTRAINT; Schema: public; Owner: postgres
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--
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ALTER TABLE ONLY public.product
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ADD CONSTRAINT product_pkey PRIMARY KEY (id);
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--
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-- PostgreSQL database dump complete
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--
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‎src/rdbms/aggregation.txt‎

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*******************************************************************************
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Data Aggregation
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*******************************************************************************
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Here is some dummy data, that will be used as example in this section.
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.. csv-table:: product
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:align: center
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:file: products_table.csv
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:header-rows: 1
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.. only:: builder_html
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The same dataset is available as:
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- :download:`csv file <./products_table.csv>`
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- :download:`postgres dump file </../assets/sql/products.pg13-1.dump.sql>`
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Aggregate functions
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===================
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*Aggregate functions* compute a single result from a set of input values.
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Various RDBMS implement various built-in aggregate functions. In this section
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we will review 5 basic functions that are available in all RDBMS.
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.. image:: /../assets/img/aggregate-function.svg
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Count
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-----
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The ``count(field)`` function returns a count of the number of times that
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``field`` is not ``NULL`` in a group.
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The ``count(*)`` function (with no arguments) returns the total number of rows
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in the group.
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.. code-block:: sql
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SELECT count(*) as "total products" FROM product;
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::
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total_products
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----------------
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10
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Maximum value
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-------------
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The ``max()`` aggregate function returns the maximum value of all values in
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the group. The maximum value is the value that would be returned last in
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an ORDER BY on the same column.
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Aggregate function returns ``NULL`` if and only if there are no non-NULL values
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in the group.
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.. code-block:: sql
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SELECT max(price) as "maximum price" FROM product;
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::
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maximum price
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---------------
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350000
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Minimum value
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-------------
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The ``min()`` aggregate function returns the minimum non-NULL value of all
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values in the group. The minimum value is the first non-NULL value that would
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appear in an ORDER BY of the column.
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Aggregate function returns ``NULL`` if and only if there are no non-NULL values
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in the group.
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.. code-block:: sql
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SELECT min(price) as "minimum price" FROM product;
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::
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minimum price
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---------------
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20000
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Average value
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-------------
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The ``avg()`` function returns the average value of all non-NULL inputs within
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a group. String and BLOB values that do not look like numbers are interpreted
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as 0. The result is always a floating point value whenever there is at least
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one non-NULL input even if all inputs are integers.
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The result is ``NULL`` if and only if there are no non-NULL inputs.
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.. code-block:: sql
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SELECT avg(price) as "average price" FROM product;
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::
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average price
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---------------------
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100500.000000000000
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Sum of values
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-------------
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The ``sum()`` aggregate functions return the sum of all non-NULL values in
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the group.
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If there are no non-NULL input rows then function returns ``NULL``.
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.. note::
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NULL is not normally a helpful result for the sum of no rows but
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the SQL standard requires it.
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SQLite implements ``total`` aggregate function, that is similar to
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``sum``, but returns ``0.0`` (always floating point) if there are
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no non-NULL input rows.
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.. code-block:: sql
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SELECT sum(sold) as "total sold items" FROM product;
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::
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total sold items
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------------------
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3750

‎src/rdbms/index.txt‎

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Relational Database
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###############################################################################
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:abbr:`RDBMS (Relational Database Management System)` is a type of software
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that helps users to create, update, manage, and access relational databases.
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Relational databases organize data into tables, which consist of rows and
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columns. Each column represents a specific attribute of the data, while each
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row represents a specific instance of that data.
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RDBMS allow users to interact with the database using
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:abbr:`SQL (Structured Query Language)`, which is a standard language for
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managing and manipulating relational databases. SQL allows users to perform
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a variety of operations on the database, including adding, updating, and
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deleting data, as well as retrieving data based on specific criteria.
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RDBMS also provide a range of features to ensure data integrity, such as
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enforcing data constraints, supporting transactions, and providing backup
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and recovery capabilities. Additionally, RDBMS systems often provide tools
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for managing the database, such as user interfaces for creating and modifying
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tables and views, and monitoring tools for optimizing database performance.
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.. toctree::
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:name: rdbms
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aggregation

‎src/rdbms/products_table.csv‎

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id,name,price,category,sold
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1,"Samsung Galaxy",100000,Phones,500
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2,"iPhone 13 Pro",120000,Phones,300
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3,"MacBook Pro",350000,Laptops,100
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4,"Lenovo ThinkPad",150000,Laptops,200
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5,"HP Printer",20000,Printers,800
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6,"Dell Monitor",50000,Monitors,400
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7,"Sony Headphones",30000,Audio,0
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8,"Bose Soundbar",70000,Audio,0
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9,"Xbox Series X",60000,Gaming,250
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10,"PlayStation 5",55000,Gaming,350

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