MySQL中查找重复值的实现

SELECT name, COUNT(*) c FROM table GROUP BY name HAVING c > 1;

SELECT varchar_col
FROM table
GROUP BY varchar_col
HAVING COUNT(*) > 1;

SELECT  *
FROM    mytable mto
WHERE   EXISTS
        (
        SELECT  1
        FROM    mytable mti
        WHERE   mti.varchar_column = mto.varchar_column
        LIMIT 1, 1
        )
ORDER BY varchar_column;

SELECT GROUP_CONCAT(id), name, COUNT(*) c
FROM documents
GROUP BY name
HAVING c > 1;

SELECT * FROM table
   WHERE field IN (
     SELECT field FROM table GROUP BY field HAVING count(*) > 1
   ) ORDER BY field;

SELECT COUNT(CONCAT(name,email)) AS tot,
       name,
       email
FROM users
GROUP BY CONCAT(name,email)
HAVING tot>1;

WITH cte AS (
  SELECT *
    ,COUNT(*) OVER(PARTITION BY col_name) AS num_of_duplicates_group
    ,ROW_NUMBER() OVER(PARTITION BY col_name ORDER BY col_name2) AS pos_in_group
  FROM table
)
SELECT *
FROM cte
WHERE num_of_duplicates_group > 1;

-- 方法一
SELECT name, COUNT(*) c FROM table GROUP BY name HAVING c > 1;

-- 方法二
SELECT varchar_col
FROM table
GROUP BY varchar_col
HAVING COUNT(*) > 1;

-- 方法三
SELECT  *
FROM    mytable mto
WHERE   EXISTS
        (
        SELECT  1
        FROM    mytable mti
        WHERE   mti.varchar_column = mto.varchar_column
        LIMIT 1, 1
        )
ORDER BY varchar_column;

-- 方法四
SELECT GROUP_CONCAT(id), name, COUNT(*) c
FROM documents
GROUP BY name
HAVING c > 1;

CREATE INDEX idx_varchar_column ON mytable (varchar_column);

SELECT t1.id, t2.name
FROM table1 t1
JOIN table2 t2 ON t1.id = t2.id;