MySQL multi-conditional query related issues

MySQL multi-condition query problem
"SELECT * from Subcatalog the WHERE name like '% $keywords% ' and Catalog = $catalog the ORDER by rank $rank limit $pages, $rows"


The above statement, how can be done when $catalog this value is empty, you can search for other data, now here Catalog = $catalog is empty when the list there is no thing come out

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------Solution--------------------
The idea of such a child, the grammar itself adjusts under:

if ($catalog = = ") {
$tiaojian = ";
}else{
$tiaojian =  "and catalog = $catalog";
}
$sql = "SELECT * from Subcatalog WHERE name like '% $keywords% ' $tiaojian  ORDER by rank $rank limit $pages, $rows"

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