About operating MySQL database in PHP_PHP tutorial

1. Semicolon exceptions

For MySQL, the first thing you must remember is that each line of commands ends with a semicolon (;), but...no It's absolutely true, and it's the same here. When a line of MySQL is inserted into PHP code, it's best to omit the trailing 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 make the PHP syntax analyzer confused, so it is better to omit them. . In this case, although the semicolon is omitted, PHP will automatically add it for you when executing the MySQL command.
There is another situation where you don’t need to add a semicolon. When you want to display the vertical arrangement of the fields you want instead of the usual horizontal arrangement, 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
The fields of the MySQL data table must have a data type defined. There are about 25 options, most of which are straightforward and require no further explanation. But there are a few that need to be mentioned.
TEXT is not a data type, although some books may say so. It should actually be "LONG VARCHAR" or "MEDIUMTEXT".
　The format of DATE data type is YYYY-MM-DD, for example: 1999-12-08. You can easily use the date function to get the current system time in this format: date("Y-m-d")
And, you can subtract between DATA data types to get the difference in days:
$age = ($current_date - $birthdate);
Set SET is a useful data type. It is somewhat similar to 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 characters
There are two kinds 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 thing that is not easy to notice is that the "%" wildcard character needs to be used together with the LIKE keyword.

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