用SQL語句,刪除掉重複項只保留一條

在幾千條記錄裡,存在著些相同的記錄,如何能用SQL語句,刪除掉重複的呢  1、查詢表中多餘的重複記錄，重複記錄是根據單個欄位（peopleId）來判斷

SELECT
    *
FROM
    people
WHERE
    peopleId IN (
        SELECT
            peopleId
        FROM
            people
        GROUP BY
            peopleId
        HAVING
            count(peopleId) > 1
    )
 

2、刪除表中多餘的重複記錄，重複記錄是根據單個欄位（peopleId）來判斷，只留有rowid最小的記錄

DELETE
FROM
    people
WHERE
    peopleName IN (
        SELECT
            peopleName
        FROM
            people
        GROUP BY
            peopleName
        HAVING
            count(peopleName) > 1
    )
AND peopleId NOT IN (
    SELECT
        min(peopleId)
    FROM
        people
    GROUP BY
        peopleName
    HAVING
        count(peopleName) > 1
)
 

3、查詢表中多餘的重複記錄（多個欄位）

SELECT
    *
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )

4、刪除表中多餘的重複記錄（多個欄位），只留有rowid最小的記錄

DELETE
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )
AND rowid NOT IN (
    SELECT
        min(rowid)
    FROM
        vitae
    GROUP BY
        peopleId,
        seq
    HAVING
        count(*) > 1
)

5、查詢表中多餘的重複記錄（多個欄位），不包含rowid最小的記錄

SELECT
    *
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )
AND rowid NOT IN (
    SELECT
        min(rowid)
    FROM
        vitae
    GROUP BY
        peopleId,
        seq
    HAVING
        count(*) > 1
)

6.消除一個欄位的左邊的第一位：

UPDATE tableName
SET [ Title ]= RIGHT ([ Title ],(len([ Title ]) - 1))
WHERE
    Title LIKE '村%'

7.消除一個欄位的右邊的第一位：

UPDATE tableName
SET [ Title ]= LEFT ([ Title ],(len([ Title ]) - 1))
WHERE
    Title LIKE '%村'

8.假刪除表中多餘的重複記錄（多個欄位），不包含rowid最小的記錄

UPDATE vitae
SET ispass =- 1
WHERE
    peopleId IN (
        SELECT
            peopleId
        FROM
            vitae
        GROUP BY
            peopleId

可能會遇到You can't specify target table '表名' for update in FROM clause這樣的錯誤，它的意思是說，不能先select出同一表中的某些值，再update這個表(在同一語句中)，即不能依據某欄位值做判斷再來更新某欄位的值。

1、資料準備

product表資料如下：

laptop表資料如下：

2、要求：#刪除所有不生產印表機廠商生產的膝上型電腦

第一步：對aptop表進行操作

操作沒有問題，按照本思路對product表進行操作

產生You can't specify target table '表名' for update in FROM clause錯誤

3、問題解決

將SELECT出的結果再通過中間表SELECT一遍，這樣就規避了錯誤。

需要注意的是，這個問題只出現於MySQL，MSSQL和Oracle不會出現此問題。