demo2, MySQL - rextester

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#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\\' is a delimiter

select version() as 'mysql version';
drop table if exists table1;
drop table if exists table2;

CREATE TABLE table1
    (`app_id` int, `cat_id` int, `approver` varchar(5), `level` int, `proj_id` int)
;
    
INSERT INTO table1
    (`app_id`, `cat_id`, `approver`, `level`, `proj_id`)
VALUES
    (1, 1, 'pm_id', 1, 731),
    (1, 2, 'dm_id', 2, 843),
    (1, 3, 'dm_id', 1, 859),
    (2, 4, 'bo_id', 1, 859)
;

CREATE TABLE table2
    (`proj_id` int, `pm_id` int, `dm_id` int, `bo_id` int)
;
    
INSERT INTO table2
    (`proj_id`, `pm_id`, `dm_id`, `bo_id`)
VALUES
    (731, 100102, 100034, 100121),
    (843, 123121, 145721, 104321),
    (859, 112312, 132434, 132435)
;

SET @sql = NULL;
SELECT
  CONCAT('CASE', GROUP_CONCAT(DISTINCT
      CONCAT(' WHEN table1.approver = ''',
      approver,
      ''' THEN table2.', approver) SEPARATOR ' '), ' END AS approver'
  ) INTO @sql
FROM table1;

SET @sql = CONCAT('SELECT table1.app_id, table1.cat_id, ', @sql, ' FROM table1 LEFT JOIN table2 ON table1.proj_id = table2.proj_id');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;


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