How to fix ERROR 1302: Conflicting declarations in MySQL

Look at your error message carefully. The full error includes the conflicting names:

ERROR 1302: Conflicting declarations: 'name1' and 'name2'

The error message will show you exactly which two identifiers are conflicting. This is your starting point for fixing the issue.

Check the parameters defined in your CREATE PROCEDURE or CREATE FUNCTION statement. List all parameter names:

CREATE PROCEDURE my_proc(
  IN param1 VARCHAR(100),
  IN param2 INT,
  OUT result VARCHAR(255)
)

Note down all parameter names so you can compare them against local variables.

Within the BEGIN...END block of your procedure, find all DECLARE statements:

BEGIN
  DECLARE param1 VARCHAR(50);  -- PROBLEM: conflicts with input parameter
  DECLARE myVar INT;
  -- procedure code here
END

Look for any DECLARE statements that use the same name as a parameter or another DECLARE in the same block.

Choose one of the conflicting names and rename it to be unique. Typically, rename the local variable (DECLARE) rather than the parameter:

CREATE PROCEDURE my_proc(
  IN param1 VARCHAR(100),
  IN param2 INT,
  OUT result VARCHAR(255)
)
BEGIN
  DECLARE param1_temp VARCHAR(50);  -- Renamed from param1
  DECLARE myVar INT;
  SET param1_temp = param1;  -- Use the renamed variable
  -- rest of procedure
END

Update all references to use the new variable name throughout the procedure.

Adopt a naming convention to prevent future conflicts. For example, prefix local variables:

CREATE PROCEDURE process_user(
  IN userId INT,
  IN userName VARCHAR(100)
)
BEGIN
  DECLARE v_tempId INT;
  DECLARE v_tempName VARCHAR(100);
  DECLARE v_result VARCHAR(255);
  -- Use v_ prefix for all local variables
END

Common conventions include prefixes like v_ (variable), l_ (local), or suffixes like _temp or _var.

After renaming, drop and recreate the procedure:

DROP PROCEDURE IF EXISTS my_proc;

CREATE PROCEDURE my_proc(
  IN param1 VARCHAR(100),
  IN param2 INT,
  OUT result VARCHAR(255)
)
BEGIN
  DECLARE param1_temp VARCHAR(50);
  -- ... rest of code
END;

Then call it to verify it works:

CALL my_proc('test', 123, @result);
SELECT @result;