任一用途

SELECT IF(field1 IS NULL or field1 = '', 'empty', field1) as field1 
from tablename

或

SELECT case when field1 IS NULL or field1 = ''
            then 'empty'
            else field1
       end as field1 
from tablename

如果你只想检查null而不想检查空字符串，那么你也可以使用ifnull()或coalesce(field1, 'empty')，但是这不适合空字符串。

使用nullif就可以达到这个目的：

SELECT ifnull(nullif(field1,''),'empty') AS field1
  FROM tablename;

工作原理：如果field是空字符串，nullif返回NULL，否则返回字段本身。这两种情况都包括（field为NULL的情况和field为空字符串的情况）。

或者，您也可以使用CASE来执行以下操作：

SELECT CASE WHEN field1 IS NULL OR field1 = '' 
       THEN 'empty' 
       ELSE field1 END AS field1
FROM tablename.

您可以在IF中使用the IFNULL function，这样会更短一些，字段名的重复也会更少。

SELECT IF(IFNULL(field1, '') = '', 'empty', field1) AS field1 
FROM tablename

您可以创建一个函数来简化此操作。

create function IFEMPTY(s text, defaultValue text)
returns text deterministic
return if(s is null or s = '', defaultValue, s);

使用：

SELECT IFEMPTY(field1, 'empty') as field1 
from tablename

通过修整和比较，我们确保还可以处理字段中的空、制表符或空格字符内容。

SELECT
    CASE
        WHEN LTRIM(RTRIM(col_1))='' or col_1 IS NULL THEN 'Not available'
        ELSE col_1
    END AS col_alias
FROM
    my_table

SELECT * FROM ( 
    SELECT  2 AS RTYPE,V.ID AS VTYPE, DATE_FORMAT(ENTDT, ''%d-%m-%Y'')  AS ENTDT,V.NAME AS VOUCHERTYPE,VOUCHERNO,ROUND(IF((DR_CR)>0,(DR_CR),0),0) AS DR ,ROUND(IF((DR_CR)<0,(DR_CR)*-1,0),2) AS CR ,ROUND((dr_cr),2) AS BALAMT, IF(d.narr IS NULL OR d.narr='''',t.narration,d.narr) AS NARRATION 
    FROM trans_m AS t JOIN trans_dtl AS d ON(t.ID=d.TRANSID)
    JOIN acc_head L ON(D.ACC_ID=L.ID) 
    JOIN VOUCHERTYPE_M AS V ON(T.VOUCHERTYPE=V.ID)  
    WHERE T.CMPID=',COMPANYID,' AND  d.ACC_ID=',LEDGERID ,' AND t.entdt>=''',FROMDATE ,''' AND t.entdt<=''',TODATE ,''' ',VTYPE,'
    ORDER BY CAST(ENTDT AS DATE)) AS ta

如果你想签入PHP，那么你应该做一些类似的事情：

$query_s =mysql_query("SELECT YOURROWNAME from `YOURTABLENAME` where name = $name");
$ertom=mysql_fetch_array($query_s);
if ('' !== $ertom['YOURROWNAME']) {
  //do your action
  echo "It was filled";
} else { 
  echo "it was empty!";
}