docs(notes): add oracle tables entry

Author: lzwjava

notes/2025-03-25-oracle-tables-en.md

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+---
+title: Identify Tables with Data in Oracle DB
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+In Oracle Database, if you’re exploring schemas and finding that some tables appear "empty" when you click on them (e.g., via a GUI tool like SQL Developer or Toad), it doesn’t necessarily mean they have no data. There could be several reasons: permissions, filters, or you’re simply looking at the wrong tables. To systematically find out which tables in a schema contain data, you can query the database metadata or inspect the tables directly. Here’s how to do it:
+
+#### 1. **Check Your Permissions**
+   - If you’re seeing empty tables, you might lack privileges to view the data. Ensure you have `SELECT` privileges on the tables in the schema.
+   - Run this to check your privileges on a specific table:
+     ```sql
+     SELECT privilege
+     FROM dba_tab_privs
+     WHERE grantee = UPPER('your_username')
+     AND table_name = UPPER('table_name');
+     ```
+     Replace `'your_username'` and `'table_name'` accordingly. If nothing shows up, ask the schema owner or DBA to grant you access.
+
+#### 2. **Query the Number of Rows in Tables**
+   - Oracle maintains statistics about tables, including row counts, in the `USER_TABLES`, `ALL_TABLES`, or `DBA_TABLES` views (depending on your access level).
+   - To see tables with data in the current schema:
+     ```sql
+     SELECT table_name, num_rows
+     FROM user_tables
+     WHERE num_rows > 0
+     ORDER BY num_rows DESC;
+     ```
+     - `USER_TABLES`: Shows tables owned by the current user.
+     - `NUM_ROWS`: Approximate number of rows (based on last statistics update).
+
+   - If you have access to another schema (e.g., via `ALL_TABLES`):
+     ```sql
+     SELECT owner, table_name, num_rows
+     FROM all_tables
+     WHERE num_rows > 0
+     AND owner = UPPER('schema_name')
+     ORDER BY num_rows DESC;
+     ```
+     Replace `'schema_name'` with the schema you’re investigating.
+
+   **Note**: `NUM_ROWS` might be outdated if statistics haven’t been recently gathered. See Step 5 to update them.
+
+#### 3. **Manually Check Specific Tables**
+   - If you suspect `NUM_ROWS` is unreliable or want to verify, run a `COUNT(*)` on individual tables:
+     ```sql
+     SELECT table_name
+     FROM user_tables;
+     ```
+     This lists all tables in your schema. Then, for each table:
+     ```sql
+     SELECT COUNT(*) FROM table_name;
+     ```
+     If the count is greater than 0, the table has data. Be cautious with large tables—`COUNT(*)` can be slow.
+
+#### 4. **Use a Script to Automate Checking**
+   - To avoid manually querying each table, use a PL/SQL script to check row counts across all tables in a schema:
+     ```sql
+     BEGIN
+         FOR t IN (SELECT table_name FROM user_tables)
+         LOOP
+             EXECUTE IMMEDIATE 'SELECT COUNT(*) FROM ' || t.table_name INTO v_count;
+             IF v_count > 0 THEN
+                 DBMS_OUTPUT.PUT_LINE(t.table_name || ' has ' || v_count || ' rows');
+             END IF;
+         END LOOP;
+     EXCEPTION
+         WHEN OTHERS THEN
+             DBMS_OUTPUT.PUT_LINE('Error on table ' || t.table_name || ': ' || SQLERRM);
+     END;
+     /
+     ```
+     - Enable output in your tool (e.g., `SET SERVEROUTPUT ON` in SQL*Plus or SQL Developer).
+     - This prints only tables with data. Adjust `user_tables` to `all_tables` and add `owner` filtering if checking another schema.
+
+#### 5. **Update Table Statistics (if Necessary)**
+   - If `NUM_ROWS` in `USER_TABLES` or `ALL_TABLES` shows 0 or seems wrong, the statistics might be stale. To update them:
+     ```sql
+     EXEC DBMS_STATS.GATHER_SCHEMA_STATS(ownname => 'schema_name');
+     ```
+     Replace `'schema_name'` with the schema name (use your username for your own schema). Then rerun the `USER_TABLES` query from Step 2.
+
+#### 6. **Check for Partitioned Tables**
+   - If the schema uses partitioned tables, the data might be spread across partitions, and a simple query might not reflect this. Check for partitions:
+     ```sql
+     SELECT table_name, partition_name, num_rows
+     FROM user_tab_partitions
+     WHERE num_rows > 0
+     ORDER BY table_name, partition_name;
+     ```
+     This shows which partitions contain data.
+
+#### 7. **GUI Tool Tips (e.g., SQL Developer)**
+   - If you’re using a GUI like Oracle SQL Developer:
+     1. Right-click the schema in the Connections pane.
+     2. Expand the "Tables" node.
+     3. Right-click a table, select "Count Rows," or open the "Data" tab to preview contents (if permitted).
+     - If the "Data" tab is empty, it could be a permissions issue or the table genuinely has no rows.
+
+---
+
+### Practical Example
+Suppose you’re in a schema called `HR`. You’d:
+1. Run:
+   ```sql
+   SELECT table_name, num_rows
+   FROM user_tables
+   WHERE num_rows > 0;
+   ```
+   Output might look like:
+   ```
+   TABLE_NAME   NUM_ROWS
+   ----------   --------
+   EMPLOYEES    107
+   DEPARTMENTS  27
+   ```
+   This tells you `EMPLOYEES` and `DEPARTMENTS` have data.
+
+2. If you suspect stale stats, update them:
+   ```sql
+   EXEC DBMS_STATS.GATHER_SCHEMA_STATS('HR');
+   ```
+3. Recheck with the same query.
+
+---
+
+### Why Tables Might Appear Empty
+- **No Data**: The table genuinely has no rows.
+- **Permissions**: You can see the table structure but not the data.
+- **Filters**: Your GUI tool might have filters hiding rows (e.g., a `WHERE` clause).
+- **Truncated/Deleted Data**: Data might have been recently removed.
+- **Synonyms/Views**: You might be looking at a synonym or view pointing to an empty or inaccessible table.
+
+If you’re still stuck, let me know your specific setup (e.g., schema name, tool used), and I can tailor the advice further!