Sample Code:

Apex Class:

public with sharing class AccountController {
 
    @AuraEnabled( cacheable = true )
    public static List< Account > fetchAccounts() {
     
        return [ SELECT Id, Name, Industry, AccountNumber, Rating, Type, Phone
                   FROM Account
                  LIMIT 100 ];
         
    }
     
}

Lightning Web Component:

HTML:

<template>
     
    <lightning-card title = "Accounts" icon-name = "custom:custom63">
 
        <div class = "slds-m-around_medium">
 
            <template if:true = {accounts}>
                 
                <div style="height: 300px;">
 
                    <lightning-datatable key-field="Id"
                                         data={accounts}
                                         columns={columns}  
                                         show-row-number-column="true"
                                         onrowselection={handleSelect}>
                    </lightning-datatable>
 
                </div>                  
     
            </template>
     
            <template if:true = {error}>
 
                {error}>
                 
            </template>
 
        </div>
 
    </lightning-card>
 
</template>

JavaScript:

import { LightningElement, wire } from 'lwc';
import fetchAccounts from '@salesforce/apex/AccountController.fetchAccounts';
 
const columns = [   
    { label: 'Name', fieldName: 'Name' },
    { label: 'Industry', fieldName: 'Industry' }
];

export default class sample extends LightningElement() {
     
    accounts;
    error;
    columns = columns;
    selectedRows = [];
 
    @wire( fetchAccounts )  
    wiredAccount( value ) {

        this.wiredRecords = value; // track the provisioned value
        const { data, error } = value;

        if ( data ) {
                            
            this.accounts = data;
            this.error = undefined;

        } else if ( error ) {

            this.error = error;
            this.accounts = undefined;

        }

    }  

    handleSelect( event ) {

        const selRows = event.detail.selectedRows;
        console.log( 'Selected Rows are ' + JSON.stringify ( selRows ) );
        if ( this.selectedRows.length < selRows.length ) {

            console.log( 'Selected' );

        } else {

            console.log( 'Deselected' );
            let deselectedRecs = this.selectedRows
                 .filter(x => !selRows.includes(x))
                 .concat(selRows.filter(x => !this.selectedRows.includes(x)));

            console.log( 'Deselected Recs are ' + JSON.stringify( deselectedRecs ) );

        }
        this.selectedRows = selRows;

    }

}

Output: