mysql开发技巧之JOIN更新和数据查重/去重

UPDATE t1 
SET col_a = 'hi' 
WHERE t1.col_a IN (
 SELECT b.col_a
 FROM t1 a INNER JOIN t2 b on
 a.col_a = b.col_a
)
;
ERROR:1093 

UPDATE t1 aa JOIN(
 SELECT b.col_a
 FROM t1 a INNER JOIN t2 b on
 a.col_a = b.col_a
)bb on aa.col_a= bb.col_a
SET col_a = 'hi' 
;

SELECT col_a, COUNT(*)
FROM t1
GROUP BY col_a HAVING COUNT(*) > 1
;

DELETE a
FROM t1 a JOIN (
 SELECT col_a,COUNT(*),MAX(id) AS id
 FROM t1
 GROUP BY col_a HAVING COUNT(*) > 1
)b ON a.col_a = b.col_a
WHERE a.id < b.id
;