MySQL语句中如何灵活对if例子进行运用
以下的文章主要描述的是MySQL语句中如何用if例子的实际操作步骤，我们先是以相关代码的方式来引出MySQL语句中如何用if例子的实际操作，以下就是文章的具体操作内容的描述，望你会有所收获。 *,if( sva = 1 ,男,女)asssvafromtanamewheresva 12.2. 控制流程函
以下的文章主要描述的是MySQL语句中如何用if例子的实际操作步骤，我们先是以相关代码的方式来引出MySQL语句中如何用if例子的实际操作，以下就是文章的具体操作内容的描述，望你会有所收获。
<ol class="dp-xml"><li class="alt"><span><span>*,if(</span><span class="attribute">sva</span><span>=</span><span class="attribute-value">1</span><span>,"男","女") as ssva from taname where sva</span><span class="tag"><span class="tag">></span><span>"" </span></span></span></li></ol>
12.2. 控制流程函数
<ol class="dp-xml"><li class="alt"><span><span>CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] <br>[ELSE result] END CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END </span></span></li></ol>
在第一个方案的返回结果中， value=compare-value。而第二个方案的返回结果是第一种情况的真实结果。如果没有匹配的结果值，则返回结果为ELSE后的结果，如果没有ELSE 部分，则返回值为 NULL。
<ol class="dp-xml"> <li class="alt"><span><span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT CASE 1 WHEN 1 THEN 'one' </span></span></li> <li> <span>-</span><span class="tag">></span><span> WHEN 2 THEN 'two' ELSE 'more' END; </span> </li> <li class="alt"> <span>-</span><span class="tag">></span><span> 'one' </span> </li> <li> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT CASE WHEN 1</span><span class="tag">></span><span>0 THEN 'true' ELSE 'false' END; </span> </li> <li class="alt"> <span>-</span><span class="tag">></span><span> 'true' </span> </li> <li> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT CASE BINARY 'B' </span> </li> <li class="alt"> <span>-</span><span class="tag">></span><span> WHEN 'a' THEN 1 WHEN 'b' THEN 2 END; </span> </li> <li> <span>-</span><span class="tag">></span><span> NULL </span> </li> </ol>
一个CASE表达式的默认返回值类型是任何返回值的相容集合类型，但具体情况视其所在语境而定。如果用在字符串语境中，则返回结果味字符串。如果用在数字语境中，则返回结果为十进制值、实值或整数值。
<ol class="dp-xml"><li class="alt"><span><span>IF(expr1,expr2,expr3) </span></span></li></ol>
如果 expr1 是TRUE (expr1 0 and expr1 NULL)，则 MySQL语句中用ifIF()的返回值为expr2; 否则返回值则为 expr3。IF() 的返回值为数字值或字符串值，具体情况视其所在语境而定。
<ol class="dp-xml">
<li class="alt"><span><span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IF(1</span><span class="tag">></span><span>2,2,3); </span></span></li>
<li>
<span>-</span><span class="tag">></span><span> 3 </span>
</li>
<li class="alt">
<span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IF(1</span><span class="tag"><span class="tag-name">2</span><span>,'yes ','no'); </span></span>
</li>
<li>
<span>-</span><span class="tag">></span><span> 'yes' </span>
</li>
<li class="alt">
<span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IF(STRCMP('test','test1'),'no','yes'); </span>
</li>
<li>
<span>-</span><span class="tag">></span><span> 'no' </span>
</li>
</ol>如果expr2 或expr3中只有一个明确是 NULL，则MySQL语句中用ifIF() 函数的结果类型 为非NULL表达式的结果类型。
expr1 作为一个整数值进行计算，就是说，假如你正在验证浮点值或字符串值， 那么应该使用比较运算进行检验。
<ol class="dp-xml"> <li class="alt"><span><span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IF(0.1,1,0); </span></span></li> <li> <span>-</span><span class="tag">></span><span> 0 </span> </li> <li class="alt"> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IF(0.1</span><span class="tag"><span class="tag">></span><span>0,1,0); </span></span> </li> <li> <span>-</span><span class="tag">></span><span> 1 </span> </li> </ol>
在所示的第一个例子中，IF(0.1)的返回值为0，原因是 0.1 被转化为整数值，从而引起一个对 IF(0)的检验。这或许不是你想要的情况。在第二个例子中，比较检验了原始浮点值，目的是为了了解是否其为非零值。比较结果使用整数。
MySQL语句中用ifIF() (这一点在其被储存到临时表时很重要 ) 的默认返回值类型按照以下方式计算：
表达式
返回值
expr2 或expr3 返回值为一个字符串。
字符串
expr2 或expr3 返回值为一个浮点值。
浮点
expr2 或 expr3 返回值为一个整数。
整数
假如expr2 和expr3 都是字符串，且其中任何一个字符串区分大小写，则返回结果是区分大小写。http://blog.knowsky.com/
IFNULL(expr1,expr2)
假如expr1 不为 NULL，则 IFNULL() 的返回值为 expr1; 否则其返回值为 expr2。IFNULL()的返回值是数字或是字符串，具体情况取决于其所使用的语境。
<ol class="dp-xml"> <li class="alt"><span><span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IFNULL(1,0); </span></span></li> <li> <span>-</span><span class="tag">></span><span> 1 </span> </li> <li class="alt"> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IFNULL(NULL,10); </span> </li> <li> <span>-</span><span class="tag">></span><span> 10 </span> </li> <li class="alt"> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IFNULL(1/0,10); </span> </li> <li> <span>-</span><span class="tag">></span><span> 10 </span> </li> <li class="alt"> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT IFNULL(1/0,'yes'); </span> </li> <li> <span>-</span><span class="tag">></span><span> 'yes' </span> </li> </ol>
IFNULL(expr1,expr2)的默认结果值为两个表达式中更加“通用”的一个，顺序为STRING、 REAL或 INTEGER。假设一个基于表达式的表的情况， 或MySQL(和PHP搭配之最佳组合)必须在内存储器中储存一个临时表中MySQL语句中用ifIFNULL()的返回值：
CREATE TABLE tmp SELECT IFNULL(1,'test') AS test；
在这个例子中，测试列的类型为 CHAR(4)。
NULLIF(expr1,expr2)
如果expr1 = expr2 成立，那么返回值为NULL，否则返回值为 expr1。这和CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END相同。
<ol class="dp-xml"> <li class="alt"><span><span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT NULLIF(1,1); </span></span></li> <li> <span>-</span><span class="tag">></span><span> NULL </span> </li> <li class="alt"> <span>MySQL(和PHP搭配之最佳组合)</span><span class="tag">></span><span> SELECT NULLIF(1,2); </span> </li> <li> <span>-</span><span class="tag">></span><span> 1 </span> </li> </ol>
注意，如果参数不相等，则 MySQL(和PHP搭配之最佳组合) 两次求得的值为 expr1 。
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