docs(notes): add ibm-db2-index-value-en notes

Author: lzwjava

Diff for: notes/2025-04-09-ibm-db2-index-value-en.md

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+---
+title: UNIQUERULE Values in IBM Db2 
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+In the IBM Db2 system catalog view `SYSCAT.INDEXES`, the `UNIQUERULE` column indicates the uniqueness property of an index. The possible values for `UNIQUERULE` are:
+
+- **`D`**: Duplicates are allowed (non-unique index).
+- **`P`**: Primary key index (unique, associated with the table's primary key constraint).
+- **`U`**: Unique index (enforces uniqueness but is not a primary key).
+
+You mentioned "D, P, D" in your question, which seems like a typo or a misinterpretation of the output. I assume you meant to ask about the values `D`, `P`, and possibly `U` appearing in the results of your query. Let me clarify what each value represents:
+
+---
+
+### `UNIQUERULE` Values Explained
+1. **`D` (Duplicates Allowed)**:
+   - The index does not enforce uniqueness.
+   - Multiple rows can have the same value(s) in the indexed column(s).
+   - Example: A regular index created without the `UNIQUE` keyword.
+   - Use case: Speeding up queries without restricting data.
+
+   **Example**:
+   ```sql
+   CREATE INDEX idx_dept ON employees (department_id);
+   ```
+   If `department_id` can have duplicates, `UNIQUERULE` will be `D`.
+
+2. **`P` (Primary Key)**:
+   - The index enforces uniqueness and is tied to the table's primary key constraint.
+   - Automatically created when you define a primary key on a table.
+   - Only one primary key index can exist per table.
+   - Use case: Ensuring a unique identifier for each row.
+
+   **Example**:
+   ```sql
+   CREATE TABLE employees (
+       employee_id INT NOT NULL,
+       PRIMARY KEY (employee_id)
+   );
+   ```
+   The index backing the primary key will have `UNIQUERULE = 'P'`.
+
+3. **`U` (Unique Index)**:
+   - The index enforces uniqueness but is not a primary key.
+   - Created explicitly with the `CREATE UNIQUE INDEX` statement.
+   - Use case: Enforcing uniqueness on columns other than the primary key (e.g., email addresses).
+
+   **Example**:
+   ```sql
+   CREATE UNIQUE INDEX idx_email ON employees (email);
+   ```
+   Here, `UNIQUERULE` will be `U`.
+
+---
+
+### Sample Output Interpretation
+Running your query:
+```sql
+SELECT 
+    INDNAME, 
+    TABSCHEMA, 
+    TABNAME, 
+    COLNAMES, 
+    UNIQUERULE, 
+    INDEXTYPE
+FROM 
+    SYSCAT.INDEXES
+WHERE 
+    TABNAME = 'EMPLOYEES'
+    AND TABSCHEMA = 'MY_SCHEMA';
+```
+
+You might see:
+```
+INDNAME        TABSCHEMA  TABNAME    COLNAMES       UNIQUERULE  INDEXTYPE
+-------------  ---------- ---------- -------------- ----------- ---------
+PK_EMPLOYEES   MY_SCHEMA  EMPLOYEES  +EMPLOYEE_ID   P           REG
+IDX_EMAIL      MY_SCHEMA  EMPLOYEES  +EMAIL         U           REG
+IDX_DEPT       MY_SCHEMA  EMPLOYEES  +DEPARTMENT_ID D           REG
+```
+
+- **`PK_EMPLOYEES`**: `UNIQUERULE = 'P'` → Primary key index on `employee_id`.
+- **`IDX_EMAIL`**: `UNIQUERULE = 'U'` → Unique index on `email`.
+- **`IDX_DEPT`**: `UNIQUERULE = 'D'` → Non-unique index on `department_id`.
+
+---
+
+### Why No Repeated "D, P, D"?
+The values `D`, `P`, and `U` appear individually for each index in the result set, not as a sequence like "D, P, D." Each row represents one index, and `UNIQUERULE` will be one of these values based on how the index was defined. If you saw something like "D, P, D" in your output, it might be a misunderstanding of multiple rows or a formatting issue in how the results were displayed.
+
+---
+
+### Quick Reference
+| UNIQUERULE | Meaning             | Created By                     |
+|------------|---------------------|--------------------------------|
+| D          | Duplicates allowed  | `CREATE INDEX` (non-unique)    |
+| P          | Primary key         | `PRIMARY KEY` constraint       |
+| U          | Unique index        | `CREATE UNIQUE INDEX`          |
+
+Let me know if you have a specific output you’d like me to interpret or if you need further clarification!