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80 lines
2.4 KiB
Markdown
80 lines
2.4 KiB
Markdown
# Oracle to PostgreSQL: NVL, DECODE, and Null-Handling Functions
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## Problem
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Oracle provides several functions — `NVL`, `NVL2`, `DECODE` — that have no direct equivalents in standard SQL but are commonly used in Oracle stored procedures and inline SQL. PostgreSQL uses standard SQL alternatives: `COALESCE`, `NULLIF`, and `CASE` expressions.
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## Behavior Comparison
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### NVL
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**Oracle:** `NVL(expr, replacement)` — returns `replacement` if `expr` is `NULL`, otherwise `expr`.
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**PostgreSQL:** Use `COALESCE(expr, replacement)` — semantically identical for two arguments.
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```sql
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-- Oracle
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NVL(column_name, 'default')
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-- PostgreSQL
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COALESCE(column_name, 'default')
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```
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### NVL2
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**Oracle:** `NVL2(expr, not_null_val, null_val)` — returns `not_null_val` if `expr` IS NOT NULL, `null_val` if NULL.
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**PostgreSQL:** No direct equivalent — use `CASE`:
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```sql
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-- Oracle
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NVL2(column_name, 'has value', 'no value')
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-- PostgreSQL
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CASE WHEN column_name IS NOT NULL THEN 'has value' ELSE 'no value' END
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```
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### DECODE
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**Oracle:** `DECODE(expr, search1, result1, search2, result2, ..., default)` — equality-based switch.
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**PostgreSQL:** No `DECODE` function — use `CASE WHEN`:
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```sql
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-- Oracle
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DECODE(status, 1, 'Active', 2, 'Inactive', 'Unknown')
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-- PostgreSQL
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CASE status
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WHEN 1 THEN 'Active'
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WHEN 2 THEN 'Inactive'
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ELSE 'Unknown'
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END
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```
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Note: `DECODE` in Oracle treats two `NULL` values as equal (unlike `=`). If any search value is `NULL`, use `IS NULL` in the `CASE` equivalent:
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```sql
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-- Oracle: DECODE treats NULL = NULL
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DECODE(col, NULL, 'empty', col)
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-- PostgreSQL
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CASE WHEN col IS NULL THEN 'empty' ELSE col END
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```
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## Migration Actions
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### 1. Stored Procedures
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Apply the direct substitutions above. Pay special attention to:
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- `NVL` on numeric expressions — `COALESCE` is type-sensitive in PostgreSQL; ensure both arguments are the same type or cast explicitly.
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- `DECODE` with `NULL` search values — replace with `IS NULL` guard in the `CASE` expression.
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### 2. Application Code (inline SQL strings)
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Search C# string literals and query builders for `NVL(`, `NVL2(`, and `DECODE(`. Apply the same substitutions.
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### 3. Tests
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Write test cases that exercise `NULL` inputs specifically — the Oracle → PostgreSQL translation of `NVL`/`COALESCE` is straightforward, but edge cases around NULL equality in `DECODE` → `CASE` are a common source of silent behavioral differences.
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