如何一次又一次地重复（递归）查询？

使用递归CTE：

with recursive cte as (
  select myCOL,
    substr(myCOL, 1, instr(myCOL, '<Tag>') - 1)
    || '■' ||      
    substr(myCOL, instr(myCOL, '</Tag>') + 6, length(myCOL)) newCOL
  from myTable
  where myCOL like '%<Tag>%'
  union all                                                     
  select c.myCOL,
    substr(c.newCOL, 1, instr(c.newCOL, '<Tag>') - 1)
    || '■' ||      
    substr(c.newCOL, instr(c.newCOL, '</Tag>') + 6, length(c.newCOL)) newCOL
  from cte c                  
  where c.newCOL like '%<Tag>%'                                                      
)  
update myTable
set myCOL = (
  select newCOL from cte
  where myTable.myCOL = cte.myCOL and cte.newCOL not like '%<Tag>%'
)           
where myCOL like '%<Tag>%';

参见演示。
结果：

| myCOL            |
| ---------------- |
| abc■def          |
| abc■def..ghi■jkl |