有几件事：
我建议在您的case语句中创建一个临时数据表，然后加入该表进行更新。
重新加入正在更新的表不起作用。
where子句中有语法错误。你在进去之前不需要等号。
尝试：

DROP TABLE IF EXISTS temp_table1;
CREATE TEMPORARY TABLE temp_table1 AS
SELECT field1,count(*) as field_count FROM table1 group by field1;

UPDATE table1 AS t1
LEFT JOIN temp_table1 aa
  ON aa.field1= t1.field1
SET t1.field1 = (CASE 
  WHEN aa.field1 = 1 AND aa.field_count < 100 THEN 1 
  WHEN aa.field1 = 2 AND aa.field_count < 100 THEN 2 
  WHEN aa.field1 = 3 AND aa.field_count < 100 THEN 3 
  WHEN aa.field1 = 4 AND aa.field_count < 100 THEN 4 
  WHEN aa.field1 = 5 AND aa.field_count < 100 THEN 5 END)
WHERE t1.id IN (100, 200, 300);

以下查询将对您有所帮助，

UPDATE table1 as t1
SET
t1.field1 = (CASE         
    WHEN (select p.cnt from (SELECT COUNT(*) as cnt FROM table1 t2 WHERE t2.field1= 1) as p) < 100 THEN 1
    WHEN (select p.cnt from (SELECT COUNT(*) as cnt FROM table1 t2 WHERE t2.field1= 2) as p) < 100 THEN 2
    WHEN (select p.cnt from (SELECT COUNT(*) as cnt FROM table1 t2 WHERE t2.field1= 3) as p) < 100 THEN 3
    WHEN (select p.cnt from (SELECT COUNT(*) as cnt FROM table1 t2 WHERE t2.field1= 4) as p) < 100 THEN 4
    WHEN (select p.cnt from (SELECT COUNT(*) as cnt FROM table1 t2 WHERE t2.field1= 5) as p) < 100 THEN 5    
 END)
WHERE t1.id IN(100, 200, 300)