invtax

### 1) Overview
 The `invtax` table is a transactional sub-ledger that stores itemized tax and duty components (such as Excise, VAT, CST, and various cesses) for sales invoices. While `invhdr` stores the total invoice value, `invtax` provides the breakdown of specific levies based on a `duty_code`.
 
 ### 2) Column Dictionary
 
 | Column | Type | Nullable | Meaning | Allowed Values | SQL Usage |
 | :--- | :--- | :--- | :--- | :--- | :--- |
 | **index** | integer | NO | Primary Key (Identity) | | Internal reference. |
 | **compcode** | string | NO | Company Identifier | e.g., "DAS" | Part of composite join key. |
 | **Sno** | integer | YES | Serial number within the invoice tax list | 1, 2, 3... | Used for ordering tax lines. |
 | **invno** | integer | YES | Sales Invoice Number | | Primary link to `invhdr` and `invdtl`. |
 | **invyear** | integer | YES | Financial Year of the invoice | e.g., 2013 | Essential for unique invoice identification. |
 | **duty_code** | integer | YES | Identifier for tax type | 1=Basic, 7=Excise, 11=VAT, 13=CST, 22=Other | Joined with `dutymas` or `cdutymas`. |
 | **taxper** | number | YES | Tax percentage rate applied | 0.00 - 100.00 | Informational for reporting. |
 | **taxamount** | number | YES | Monetary value of this specific tax/duty | | Aggregated for total tax reports. |
 
 ### 3) Relationships & Join Map
 
 #### Parent Table (Authority)
 * **dbo.invhdr**: Joins on `compcode`, `invno`, and `invyear`. This is the most common join used to filter taxes by date or customer.
 * **dbo.invdtl**: Relates via `invno`, `invyear`, and `compcode` to correlate specific tax lines with itemized goods.
 
 #### Master Data Reference
 * **dbo.dutymas** / **dbo.cdutymas**: **Likely** joins on `duty_code` and `compcode` to retrieve the human-readable name of the tax (e.g., "VAT @ 12.5%").
 
 #### Join Map Table
 | Target Table | Join Columns | Join Type | Logic |
 | :--- | :--- | :--- | :--- |
 | `dbo.invhdr` | `compcode`, `invno`, `invyear` | One-to-Many | Links tax breakdown to invoice header (dates/customers). |
 | `dbo.invdtl` | `compcode`, `invno`, `invyear` | Many-to-Many | Connects tax components to specific invoice items. |
 | `dbo.dutymas` | `compcode`, `duty_code` | Many-to-One | **Likely** lookup for tax names and accounting ledgers. |
 | `dbo.invdesphdr`| `compcode`, `invno`, `invyear` | One-to-Many | **Likely** links to dispatch/delivery documents. |
 
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