How do you use the CONCAT function in SQL?

The CONCAT function in SQL is used to combine two or more strings into one string.

The CONCAT function is a built-in function in SQL that allows you to concatenate, or join, two or more strings together. This function is particularly useful when you need to combine data from different columns or strings to create a new, single string. The syntax for using the CONCAT function is straightforward: you simply write "CONCAT", followed by the strings you want to combine in parentheses, separated by commas.

For example, if you have a table of customers with separate columns for first name and last name, and you want to create a full name column, you could use the CONCAT function like this:

```
SELECT CONCAT(first_name, ' ', last_name) AS 'Full Name'
FROM customers;
```

In this example, 'first_name' and 'last_name' are the names of the columns you're combining, and ' ' is a space character that separates the first name and last name in the new 'Full Name' column. The AS keyword is used to rename the new column.

It's important to note that the CONCAT function can take any number of arguments, and the arguments can be any string value, including column names, string literals, or the result of another function. For instance, you could use CONCAT to combine a static string with a column value, like this:

```
SELECT CONCAT('Customer: ', first_name) AS 'Customer Name'
FROM customers;
```

In this case, the static string 'Customer: ' is combined with the value in the 'first_name' column for each row in the 'customers' table.

Remember, the CONCAT function will return a new string that is the result of combining all the input strings. If any of the input strings is NULL, then CONCAT will treat it as an empty string. This is a handy feature, as it allows you to use CONCAT without worrying about NULL values causing errors in your SQL queries.