What issues should you pay attention to when operating MySQL in PHP_PHP Tutorial

1. Semicolon exceptions

In MySQL, each line of commands ends with a semicolon (;), but when a line of MySQL commands is inserted into PHP code, it is best to omit the following semicolon, for example:

mysql_query ("INSERT INTO tablename (first_name, last_name) VALUES ('$first_name', '$last_name')");

This is because PHP also ends a line with a semicolon. Extra semicolons sometimes cause PHP's syntax analyzer to make incorrect analysis, so it is better to omit them. In this case, although the semicolon is omitted, PHP will automatically add it when executing the MySQL command.

There is another situation where you don’t need to add a semicolon. When you want to display the fields you want arranged vertically instead of horizontally as usual, you can use G to end a line of SQL statements. In this case, there is no need for a semicolon, for example:

SELECT * FROM PENPALS WHERE USER_ID = 1G

2. TEXT, DATE, and SET data types

TEXT is not a data type, it should be "LONG VARCHAR" or "MEDIUMTEXT".

The format of the DATE data type is YYYY-MM-DD, for example: 2001-10-01. You can easily use the date function to get the current system time in this format:

date("Y-m-d")

Moreover, you can subtract between DATA data types to get the difference in time and days:

　　$age = ($current_date - $birthdate);

The set SET is a useful data type. It is somewhat similar to the enumeration ENUM, except that SET can save multiple values ​​and ENUM can only save one value. Moreover, the SET type can only have a maximum of 64 predefined values, while the ENUM type can handle a maximum of 65,535 predefined values. And what if we need a collection with more than 64 values? At this time, you need to define multiple collections to solve this problem together.

3. Wildcard

There are two types of wildcard characters in SQL: " * " and " % ". Used in different situations. For example: If you want to see all the contents of the database, you can query like this:

SELECT * FROM dbname WHERE USER_ID LIKE '%';

Here, both wildcards are used. They mean the same thing - they both match any string - but they are used in different contexts. " * " is used to match field names, and " % " is used to match field values. Another point that is not easy to attract attention is that the "%" wildcard needs to be used together with the LIKE keyword.

There is also a wildcard character, which is the underscore "_". It has a different meaning from the above and is used to match any single character.

4. NOT NULL and empty records

What happens if the user presses the submit button without filling in anything? If you really need a value, you can use client-side script or server-side script for data validation, as mentioned earlier. However, in the database, some fields are allowed to be left blank and nothing is filled in. For such records, MySQL will perform the following steps:

Insert the value NULL , which is the default operation.

If you declare NOT NULL for it in the field definition (when creating or modifying this field), MySQL will leave this field empty and fill it with nothing.

For a field of ENUM enumeration type, if you declare NOT NULL for it, MySQL will insert the first value of the enumeration set into the field. In other words, MySQL uses the first value in the enumeration set as the default value for this enumeration type.

There are some differences between a NULL record and an empty record. The % wildcard character can match empty records, but it cannot match NULL records. At some point, this distinction can have unintended consequences. As a rule of thumb, any field should be declared NOT NULL . In this way, the following SELECT query statement can run normally:

if (!$CITY) {$CITY = "%";}
​$selectresult = mysql_query ("SELECT * FROM dbname
​WHERE FIRST_NAME = ' Bill '
AND LAST_NAME = ' Gates '
AND CITY LIKE '$CITY'
");

In the first line, if the user does not specify a CITY value, the wildcard character % will be used to substitute the CITY variable, so that any CITY value will be taken into account during the search, even those records with an empty CITY field.

But if there are some records whose CITY field value is NULL, then a problem arises. The above query cannot find these fields. A solution to the problem could be this:

if (!$CITY) {$CITY = "%";}
​$selectresult = mysql_query ("SELECT * FROM dbname
​WHERE FIRST_NAME = ' Bill '
AND LAST_NAME = ' Gates '
​AND (CITY LIKE '$CITY' OR CITY IS NULL)
");

Note that when searching for NULL, you must use the "IS" keyword, and LIKE will not work properly.

The last thing to mention is that if you already have some records in the database before adding or modifying a new field, the value of the newly added field in the original record may be NULL or NULL. NULL, in which case use SELECT queries with extreme caution.

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