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When to surround SQL fields with apostrophes?

I notice that when I value INSERTboth SELECTin and out of the database, I have to surround the fields with single quotes, for example:

mysql_query("INSERT INTO employees (name, age) VALUES ('$name', '$age')");

However, if I were updating the age, I would not use single quotes:

mysql_query("UPDATE employees SET age = age + 1 WHERE name = '$name'");

Also, it seems that when adding a date to the SQL database, I don't need to surround it with single quotes:

mysql_query("INSERT INTO employees (name, date) VALUES ('$name', NOW())");

In addition, when using operators such as CONCAT, this is also not necessary:

mysql_query("UPDATE employees SET name=CONCAT(name,$lastName) WHERE id='$id'");

Maybe I’m just coding poorly, but it seems like I remember if I didn’t surround the single-quoted box when inserting and selecting its operation.

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4 answers

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String values ​​(including single characters) must be enclosed in single quotes. This includes date constants represented using strings. Numeric values ​​do not need quotation marks.

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