invhdr

### 1. Overview
 The `invhdr` table serves as the primary header repository for all sales invoices within the system. It tracks critical financial data (basic amounts, GST components), logistical details (vehicle numbers, removal times, E-way bill numbers), and compliance information (IRN, QR codes). It acts as the "parent" table to `invdtl`, forming the core of the Sales and Distribution module.
 
 ### 2. Column Dictionary
 | Column | Type | Nullable | Meaning | Allowed Values | SQL Usage |
 | :--- | :--- | :--- | :--- | :--- | :--- |
 | **index** | integer | No | Internal primary key. | Auto-incremented | Primary Key |
 | **compcode** | string | No | Unique identifier for the company/branch. | e.g., 'DAS', 'HFL' | Join, Filter |
 | **invtype** | integer | No | Categorization of the invoice. | 0 (Domestic), 1 (RM Sale), 2 (Scrap), 4 (Others) | Filter |
 | **invno** | integer | No | Physical Invoice Number. | Unique with Year | Join, Partition |
 | **invyear** | integer | No | Financial/Accounting year of the invoice. | e.g., 2024 | Join, Partition |
 | **invdate** | datetime | No | Date the invoice was generated. | | Range Filter |
 | **custcode** | string | No | Reference code for the customer. | Joins to `cust` | Join (custcode) |
 | **oano** | integer | Yes | Order Acknowledgement Number reference. | Joins to `oahdr` | Join (oano) |
 | **cancel_flag** | string | Yes | Status flag indicating if the invoice is void. | 'N' (Active), 'Y' (Cancelled) | Filter |
 | **basic_amt** | number | Yes | Taxable base value of goods. | | Aggregation |
 | **inv_amt** | number | Yes | Grand total including taxes and charges. | | Aggregation |
 | **cgst_amt** | number | Yes | Central GST amount. | | Financial Reporting |
 | **sgst_amt** | number | Yes | State GST amount. | | Financial Reporting |
 | **igst_amt** | number | Yes | Integrated GST amount. | | Financial Reporting |
 | **IRN** | string | Yes | Invoice Reference Number for E-Invoicing compliance. | 64-character hash | Compliance |
 | **EwayBillNo** | string | Yes | Logistics compliance number for transport. | | Compliance |
 | **totdespqty** | number | Yes | Total quantity of items dispatched in this invoice. | | Reporting |
 | **adviceno** | number | Yes | Reference to the dispatch advice. | Joins to `despadvicehdr` | Join (adviceno) |
 | **sfrm** | string | Yes | Sale Source identifier. | 'C', 'R', 'S' | Filter |
 | **invdoc** | string | Yes | Formatted document string for printing. | e.g., '2024-25/00001' | UI/Display |
 
 ### 3. Relationships & Join Map
 
 #### Authoritative Joins (Based on SCHEMA_MAP)
 * **invdtl**: Primary child relationship. Joined on `invno`, `invyear`, and `compcode`.
 * **cust**: Joins on `custcode` and `compcode` to retrieve customer master details (name, GSTIN, address).
 * **invtax**: Joins on `invno`, `invyear`, and `compcode` to retrieve granular duty/tax breakdowns.
 * **oahdr**: Joins on `oano` and `compcode` to link the invoice back to the sales order acknowledgement.
 * **despadvicehdr**: Joins on `adviceno` and `compcode` to trace the invoice back to its dispatch advice origin.
 * **invtype**: Joins on `invtype` and `compcode` to get descriptive names for invoice categories.
 * **suppmas**: Contextual Join (Likely). When `invtype = 2` (Scrap/Returns), `custcode` may link to `supcode` in `suppmas`.
 
 #### Logical Usage (Stored Procedures)
 * **Inventory Tracking**: The `FGStock` and `WIPStockSFR` procedures use `invhdr` (joined with `invdtl`) to subtract quantities from current stock balances during specific date ranges.
 * **GST Reporting**: `GstRegisterOutPut` aggregates `cgst_amt`, `sgst_amt`, and `igst_amt` from this table for tax filing reports.
 * **E-Invoicing API**: `E_Invoice_JsonAPI` extracts address, GSTIN, and IRN fields to construct the JSON payload for government portals.