awesome-copilot/skills/reviewing-oracle-to-postgres-migration/references/empty-strings-handling.md

Oracle to PostgreSQL: Empty String Handling Differences

Problem

Oracle automatically converts empty strings ('') to NULL in VARCHAR2 columns. PostgreSQL preserves empty strings as distinct from NULL. This difference can cause application logic errors and test failures during migration.

Behavior Comparison

Oracle:

• Empty string ('') is always treated as NULL in VARCHAR2 columns
• WHERE column = '' never matches rows; use WHERE column IS NULL
• Cannot distinguish between explicit empty string and NULL

PostgreSQL:

• Empty string ('') and NULL are distinct values
• WHERE column = '' matches empty strings
• WHERE column IS NULL matches NULL values

Code Example

-- Oracle behavior
INSERT INTO table (varchar_column) VALUES ('');
SELECT * FROM table WHERE varchar_column IS NULL;  -- Returns the row

-- PostgreSQL behavior  
INSERT INTO table (varchar_column) VALUES ('');
SELECT * FROM table WHERE varchar_column IS NULL;  -- Returns nothing
SELECT * FROM table WHERE varchar_column = '';     -- Returns the row

Migration Actions

1. Stored Procedures

Update logic that assumes empty strings convert to NULL:

-- Preserve Oracle behavior (convert empty to NULL):
column = NULLIF(param, '')

-- Or accept PostgreSQL behavior (preserve empty string):
column = param

2. Application Code

Review code that checks for NULL and ensure it handles empty strings appropriately:

// Before (Oracle-specific)
if (value == null) { }

// After (PostgreSQL-compatible)
if (string.IsNullOrEmpty(value)) { }

3. Tests

Update assertions to be compatible with both behaviors:

// Migration-compatible test pattern
var value = reader.IsDBNull(columnIndex) ? null : reader.GetString(columnIndex);
Assert.IsTrue(string.IsNullOrEmpty(value));

4. Data Migration

Decide whether to:

• Convert existing NULL values to empty strings
• Convert empty strings to NULL using NULLIF(column, '')
• Leave values as-is and update application logic