How to Check if value exists in a MySQL database

问题

Suppose I have this table:

id | name | city
------------------
1  | n1   | c1
2  | n2   | c2
3  | n3   | c3
4  | n4   | c4

I want to check if the value c7 exists under the variable city or not.

If it does, I will do something.
If it doesn't, I will do something else.

回答1:

preferred way, using MySQLi extension:

$mysqli = new mysqli(SERVER, DBUSER, DBPASS, DATABASE);
$result = $mysqli->query("SELECT id FROM mytable WHERE city = 'c7'");
if($result->num_rows == 0) {
     // row not found, do stuff...
} else {
    // do other stuff...
}
$mysqli->close();

deprecated:

$result = mysql_query("SELECT id FROM mytable WHERE city = 'c7'");
if(mysql_num_rows($result) == 0) {
     // row not found, do stuff...
} else {
    // do other stuff...
}

回答2:

For Exact Match

"SELECT * FROM yourTable WHERE city = 'c7'"

For Pattern / Wildcard Search

"SELECT * FROM yourTable WHERE city LIKE '%c7%'"

Of course you can change '%c7%' to '%c7' or 'c7%' depending on how you want to search it. For exact match, use first query example.

PHP

$result = mysql_query("SELECT * FROM yourTable WHERE city = 'c7'");
$matchFound = mysql_num_rows($result) > 0 ? 'yes' : 'no';
echo $matchFound;

You can also use if condition there.

回答3:

Assuming the connection is established and is available in global scope;

//Check if a value exists in a table
function record_exists ($table, $column, $value) {
    global $connection;
    $query = "SELECT * FROM {$table} WHERE {$column} = {$value}";
    $result = mysql_query ( $query, $connection );
    if ( mysql_num_rows ( $result ) ) {
        return TRUE;
    } else {
        return FALSE;
    }
}

Usage: Assuming that the value to be checked is stored in the variable $username;

if (record_exists ( 'employee', 'username', $username )){
    echo "Username is not available. Try something else.";
} else {
    echo "Username is available";
}

回答4:

SELECT
    IF city='C7'
    THEN city
    ELSE 'somethingelse'
    END as `city`
FROM `table` WHERE `city` = 'c7'

回答5:

I tried to d this for a while and $sqlcommand = 'SELECT * FROM database WHERE search="'.$searchString.'";'; $sth = $db->prepare($sqlcommand); $sth->execute(); $record = $sth->fetch(); if ($sth->fetchColumn() > 0){} just works if there are TWO identical entries, but, if you replace if ($sth->fetchColumn() > 0){} with if ($result){} it works with only one matching record, hope this helps.

回答6:

For Matching the ID:

Select * from table_name where 1=1

For Matching the Pattern:

Select * from table_name column_name Like '%string%'

来源：https://stackoverflow.com/questions/11292468/how-to-check-if-value-exists-in-a-mysql-database

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php

mysql

exists