Implemented the DECLARE VARIABLE feature for Oracle variable declarations.

[KeilorRodriguez]

Add DECLARE VARIABLE support for Oracle variable declarations (#762 )

Summary

This PR introduces support for Oracle-style variable declarations using the DECLARE VARIABLE syntax in the transpiler. It enhances compatibility with Databricks by mapping Oracle types to their appropriate equivalents and ensures backward compatibility with existing functionality.

Problem Solved

Before:
p_date_start NUMBER; → p_date_start AS NUMBER

After:
p_date_start NUMBER; → DECLARE VARIABLE p_date_start BIGINT

Implementation Details

• Added _oracle_variable_declaration_sql() to databricks.py to detect Oracle variable declarations.
• Integrated the function into the TRANSFORMS dictionary to handle exp.Alias expressions.
• Expanded Oracle-to-Databricks type mappings for comprehensive coverage.

Oracle Type Mappings

Oracle Type	Databricks Type
NUMBER, INTEGER, INT	BIGINT
VARCHAR2, VARCHAR, CHAR, NCHAR, NVARCHAR2	STRING
DATE	DATE
TIMESTAMP, TIMESTAMPTZ, TIMESTAMPLTZ	TIMESTAMP
DECIMAL, NUMERIC	DECIMAL
FLOAT, DOUBLE	DOUBLE
CLOB, NCLOB	STRING
BLOB, RAW	BINARY

Testing

• ✅ Created unit tests in test_oracle_variable_declarations.py
• ✅ Added functional test files:
  • test_variable_declarations/
  • test_variable_declarations_1.sql
  • test_variable_declarations_1.databricks.sql
• ✅ All 488 existing transpiler tests pass
• ✅ All new Oracle variable declaration tests pass

Key Features

• Smart Detection: Only converts Oracle variable declarations, not regular aliases.
• Comprehensive Type Mapping: Supports all major Oracle data types.
• Backward Compatible: No regressions in existing functionality.
• Well Tested: 12 new test cases plus full test suite coverage.

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Closes #762

[KeilorRodriguez]

@sundarshankar89 can I get somebody to review this PR?

[sundarshankar89]

@KeilorRodriguez Thanks for the PR, we don't use SQLGLOT extensions for code conversion, I will assign 762 to our developers to fix.

[sundarshankar89]

Closing this PR as we have a ticket to address this.