How to query temporary tables in Oracle database

In Oracle database, querying temporary tables is a frequent requirement. Temporary tables refer to temporary data tables stored in the database, usually used to store temporary data or intermediate result sets. Compared with traditional data tables, the life cycle of temporary tables is shorter and usually only exists in the current transaction or current connection. Once the transaction or connection ends, the temporary table will no longer exist.

In practical applications, temporary tables play a vital role in the data processing process. For example, we may need to store data in a temporary table for subsequent data processing operations, or we may need to aggregate data from multiple data tables for calculations. In this case, temporary tables are a very useful tool.

So, how to query the temporary table in the Oracle database? Below we will introduce two methods.

Method 1: Use global temporary table

Global temporary table is a mechanism provided by Oracle database for creating temporary tables that can be shared by multiple concurrent connections. Global temporary tables can share data across multiple sessions, making it a very convenient way to query temporary tables.

First, we need to create a global temporary table. The syntax for creating a global temporary table is as follows:

CREATE GLOBAL TEMPORARY TABLE temp_table
(
  id NUMBER,
  name VARCHAR2(20)
)
ON COMMIT PRESERVE ROWS;

In the above syntax, temp_table is the name of the global temporary table, id and name are the tables Columns in ON COMMIT PRESERVE ROWS means that the data in the table will not be cleared at the end of the transaction or connection.

Next, we can use the following statement to insert data into the global temporary table:

INSERT INTO temp_table VALUES (1, 'Tom');
INSERT INTO temp_table VALUES (2, 'Jerry');

Finally, we can use the following statement to query the data in the global temporary table:

SELECT * FROM temp_table;

Method 2: Use ordinary temporary tables

Ordinary temporary tables are another temporary table implementation method in Oracle database. Unlike global temporary tables, ordinary temporary tables only exist in the current session and cannot be shared by other sessions. Although this method is slightly more cumbersome than the global temporary table, it is also a feasible method to query the temporary table.

First, we need to use the following statement to create a normal temporary table in the current session:

CREATE PRIVATE TEMPORARY TABLE temp_table
(
  id NUMBER,
  name VARCHAR2(20)
);

In the above syntax, temp_table is the name of the temporary table, id and name are columns in the table.

Then, we can use the following statement to insert data into the temporary table:

INSERT INTO temp_table VALUES (1, 'Tom');
INSERT INTO temp_table VALUES (2, 'Jerry');

Finally, we can use the following statement to query the data in the temporary table:

SELECT * FROM temp_table;

Note What's more, in the creation statement of an ordinary temporary table, we need to use the PRIVATE TEMPORARY keyword to specify that an ordinary temporary table is created, not a global temporary table.

Summary

Querying temporary tables is a very common requirement in database development. In Oracle database, we can use global temporary tables or ordinary temporary tables to achieve this requirement. Global temporary tables can be shared by multiple concurrent connections, which is relatively convenient; while ordinary temporary tables only exist in the current session and need to be cleared manually, which is a bit cumbersome. When choosing a specific implementation method, you need to make a flexible choice based on actual needs and business scenarios.

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