Postgre報錯：ERROR: UNION types numeric and character varying cannot be matched

　　今天遇到一個問題，Postgre的一個查詢功能突然報錯“ERROR:  UNION types numeric and character varying cannot be matched，LINE 6: ...ipematerial,b.geom,b.pressurerating,b.pipelength, pipediam,b...”，在對兩個結果集進行合並查詢時報錯，數字和字符串的類型不能合並，SQL如下：

SELECT
    a.gid AS gid,
    a. NAME AS NAME,
    a.pipematerial,
    a.geom,
    a.pressurerating,
    a.pipelength,
    a.pipediam,
    a.wallthickness,
    a.source,
    a.target
FROM
    zy a
WHERE
    a.projectId = '2c91808f7cc0e652017ccfc7f7ea016f'
UNION ALL
    SELECT
        b.id AS gid,
        b. NAME AS NAME,
        b.pipematerial,
        b.geom,
        b.pressurerating,
        b.pipelength,
        b.pipediam,
        b.wallthickness,
        b.source,
        b.target
    FROM
        digital_edit_pipeline b
    WHERE
        b.condition_id = '2c9180887d45f311017d55857e9b02ac'
    AND b. ENABLE = 1

　　查了一下，是digital_edit_pipeline 表的pipediam字段被改成了字符串類型，這是另外一個同事出於某種原因進行了修改，導致這個地方報錯了，解決也很簡單，把兩個表的這個字段轉換成相同的數據類型就可以了，用to_number (obj, '99999.999')就可以把字符串轉數字，修改后的SQL如下：

SELECT
    a.gid AS gid,
    a. NAME AS NAME,
    a.pipematerial,
    a.geom,
    a.pressurerating,
    a.pipelength,
    a.pipediam,
    a.wallthickness,
    a.source,
    a.target
FROM
    zy a
WHERE
    a.projectId = '2c91808f7cc0e652017ccfc7f7ea016f'
UNION ALL
    SELECT
        b.id AS gid,
        b. NAME AS NAME,
        b.pipematerial,
        b.geom,
        b.pressurerating,
        b.pipelength,
        to_number (b.pipediam, '99999.999') AS pipediam,
        b.wallthickness,
        b.source,
        b.target
    FROM
        digital_edit_pipeline b
    WHERE
        b.condition_id = '2c9180887d45f311017d55857e9b02ac'
    AND b. ENABLE = 1

　　如上，就可以解決之前的報錯了。如果需要截取字符串后再轉數字，可以用trim()函數，如trim(both 'abc' from pipediam)，可以去除pipediam字段中包含abc的字符串，完整的書寫就是to_number(trim(both 'abc' from pipediam), '9999.999') AS pipediam。

　　其實不管是Postgre還是MySQL、Oracle等其他數據，這個解決思路都是一樣，差別可能就是每個數據庫的函數不太一樣。