text: -- join6.test
  -- 
  -- execsql {
  --     SELECT t1.a, t3.b 
  --       FROM t1 LEFT JOIN t2 ON t1.a=t2.a LEFT JOIN t3 ON t2.a=t3.a;
  -- }
  SELECT t1.a, t3.b 
  FROM t1 LEFT JOIN t2 ON t1.a=t2.a LEFT JOIN t3 ON t2.a=t3.a;