Inserting into a Table Where Not Exists: A Comparison of Postgres and Oracle

Introduction

When working with databases, it’s often necessary to insert data into a table where no existing records match certain conditions. The INSERT INTO ... WHERE NOT EXISTS syntax allows you to achieve this in a single statement. However, the implementation can vary significantly between different database systems, such as Postgres and Oracle.

In this article, we’ll explore how to create an INSERT INTO ... WHERE NOT EXISTS query that works on both Postgres and Oracle, highlighting any differences and potential workarounds.

Background

The NOT EXISTS clause is a useful tool for identifying records in a table that don’t match a certain condition. When used with INSERT INTO, it allows you to insert new data into a table while ensuring that the new record doesn’t duplicate an existing one with matching values.

However, not all database systems support this syntax directly. Oracle, for instance, requires a more complex approach due to its specific syntax and features.

Postgres Approach

In Postgres, the INSERT INTO ... WHERE NOT EXISTS query is relatively straightforward:

insert into table_name (col1, col2)
select 'A', 100
from dual
where not exists(select * 
                 from table_name 
                 where (col1='A' and col2=100));

The dual table in Postgres serves as a dummy table that always returns a single row. This allows us to use the NOT EXISTS clause without needing an explicit FROM statement.

Oracle Approach

Oracle, on the other hand, requires a different approach due to its specific syntax and features:

insert into table_name (col1, col2)
select 'A', 100
from dual
where not exists(
         select 1
         from table_name
         where col1='A'
         AND    col2=100
       );

As you can see, Oracle uses a different syntax for the NOT EXISTS clause. Instead of using SELECT *, we use SELECT 1. This is because Oracle doesn’t support the * wildcard in the same way as Postgres.

Creating a Dummy Table: The PostgreSQL Solution

One potential workaround for using the INSERT INTO ... WHERE NOT EXISTS syntax in both Postgres and Oracle is to create a dummy table that serves as an equivalent to the dual table in Postgres. In Postgres, you can do this with the following query:

CREATE VIEW dual(dummy) AS
  SELECT dummy FROM (VALUES('X')) AS dual(dummy);

However, creating a custom dummy table might not be the most efficient solution for every use case.

Using Views and MERGE Statements

A more efficient approach is to create a view in Postgres that serves as an equivalent to the dual table. Here’s how you can do it:

CREATE VIEW dual_view AS
SELECT 'X'::text AS dummy;

Then, in both Oracle and Postgres, you can use the following query:

INSERT INTO table_name (col1, col2)
FROM   DUAL
WHERE  NOT EXISTS(
         SELECT 1
         FROM   table_name
         WHERE  col1='A'
         AND    col2=100
       );

Or, in Oracle, you can use a MERGE statement to achieve the same result:

MERGE INTO table_name dst
USING (
  SELECT 'A' AS col1, 100 AS col2 FROM dual
) src
ON (src.col1 = dst.col1 AND src.col2 = dst.col2)
WHEN NOT MATCHED THEN
  INSERT (col1, col2)
  VALUES (src.col1, src.col2);

Oracle 23 and the Optional FROM Clause

Starting with Oracle 23, the SELECT statement is no longer required to specify a FROM clause. You can use the following query in both Postgres and Oracle:

INSERT INTO table_name (col1, col2)
SELECT 'A', 100;

This query achieves the same result as before without requiring an explicit DUAL or dual_view table.

Conclusion

When working with databases, it’s essential to be aware of the differences between various database systems and how to implement common queries in different ways. In this article, we explored how to create an INSERT INTO ... WHERE NOT EXISTS query that works on both Postgres and Oracle, highlighting potential workarounds and solutions for different use cases.

By understanding these differences, you can write more efficient and effective SQL queries that work across multiple database systems.

Last modified on 2023-11-23