short preview with mysql pdo - php

Question

I can't make it work the short preview with LEFT() and substr() only with RIGHT() with mysql PDO but I need show the first 100 characters of description not the last ones...where is the error in my code?
Code:
<?php
$sql = $conn->prepare("SELECT id_blog, titulo, LEFT(blog,100) AS blog, DATE_FORMAT(f_blog, '%d %M %Y') AS f_blog FROM BLOG ORDER BY id_blog DESC");
$sql->execute();
while($row = $sql->fetch(PDO::FETCH_ASSOC)){ echo '
<div class="span9 pad15" id="'.$row["id_blog"].'">
<div class="row">
<div class="span1">
<div class="btn btn-medium btn-rounded btn-blog1">'; echo $row["f_blog"]; echo
'</div>
</div>
<div class="span8">
<h1 class="post_link">'; echo $row["blog_titulo"]; echo '</h1>
<p>'; echo $row["blog"]; echo ' ...</p>
<div class="read_more">Leer más →</div>
<div class="pad30"></div>
</div>
</div>
</div>'; } ?>
with LEFT(blog, 100) AS blog not show nothing
with substr(blog, 0,100) AS blog not show nothing
with RIGHT(blog,100) AS blog show the last 100 characters
Here the LEFT() is working nice in the same web page but in the first one no work.
CODE:
<?php
$sql = "SELECT id_oferta, oferta_titulo, oferta_subtitulo, LEFT(oferta_mensaje, 50) AS oferta_mensaje, oferta_precio, oferta_foto FROM OFERTAS ORDER BY id_oferta DESC";
$result = $conn->query($sql);
while($row = $result->fetch(PDO::FETCH_ASSOC)) { ?>
<?php if (!empty($row['id_oferta'])) { echo '
<div class="span3" id"'.$row["id_oferta"].'">
<div class="tile">
<div>
<img src="../admin/assets/img/ofertas/'.$row["oferta_foto"].'" alt="'.$row["oferta_titulo"].'" class="img-circle" style="max-width:100px; max-height:100px;" />
</div>
<span>'; echo $row["oferta_titulo"]; echo '</span>
<h6><small>'; echo $row["oferta_subtitulo"]; echo '</small></h6>
<p>'; echo $row["oferta_mensaje"]; echo '</p>
<h5><div class="intro-icon-disc"><b>$'; echo $row["oferta_precio"]; echo '</b></div></h5>
</div>
<div class="pad25"></div>
</div>'; } ?>
<div id="myModal" class="modal hide fade" id"<?php echo $row["id_oferta"]; ?>" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true"></button>
<h3 id="myModalLabel"><b>Cupón para : <?php echo $row["oferta_titulo"]; ?></b></h3>
</div>
<div class="modal-body">
<form name="cupon" id="cupon" method="post">
<div class="row-fluid grid">
<div class="span6">
<label><b>Nombres : </b></label><input type="text" class="input-block-level" name="nombres" />
</div>
<div class="span2">
</div>
<div class="span4">
<label><b>DUI : </b></label><input type="text" class="input-block-level" name="dui" />
</div>
</div>
<div class="row-fluid grid">
<div class="span6">
<label><b>Correo : </b></label><input type="text" class="input-block-level" name="corelectronico" />
</div>
</div>
<input type="hidden" name="of_titulo" type="text" value="<?php echo $row['oferta_titulo']; ?>" />
<input type="hidden" name="of_stitulo" type="text" value="<?php echo $row['oferta_subtitulo']; ?>" />
<input type="hidden" name="of_mensaje" type="text" value="<?php echo $row['oferta_mensaje']; ?>" />
<input type="hidden" name="of_precio" type="text" value="<?php echo $row['oferta_precio']; ?>" />
<input type="hidden" name="of_foto" type="text" value="<?php echo $row['oferta_foto']; ?>" />
<div class="row-fluid grid">
<div class="span4" id="div1">
<input type="hidden" name="action" value="create_cupon" />
<button class="positive btn btn-inverse" type="submit" id="enviar" name="enviar">
<i class="icon icon-save icon-white"></i>
Grabar Cupón
</button>
</div>
<div class="span4">
</div>
<div class="span4" id="div2">
<a class="negative btn btn-info" href="impcupon.php" target="popup" onClick="window.open(this.href, this.target, "scrollbars=yes,toolbar=no,menubar=no,status=no,width=parent,height=parent"); return false;" onclick="boton(this)" id="imprimir">
<i class=" icon-print icon-white"></i>
Imprimir Cupón
</a>
</div>
</div>
</form>
<div id="loading_cupon" style="display:none;"><img src="img/loaders/loader.gif" /></div>
</div>
<div class="modal-footer">
<button class="btn" data-dismiss="modal" aria-hidden="true">
Cerrar
</button>
</div>
</div><?php } ?>

Answer 1

As mentioned in your comments oferta_mensaje is type longtext. You can use CAST() in this case.
"SELECT id_oferta, oferta_titulo, oferta_subtitulo,
CAST(oferta_mensaje as char(50))
AS oferta_mensaje, oferta_precio, oferta_foto
FROM OFERTAS ORDER BY id_oferta DESC";