3.2 Entering Queries

Make sure that you are connected to the server, as discussed in the previous section. Doing so does not in itself select any database to work with, but that is okay. At this point, it is more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering queries, using several queries you can try out to familiarize yourself with how mysql works.

Here is a simple query that asks the server to tell you its version number and the current date. Type it in as shown here following the mysql> prompt and press Enter:

This query illustrates several things about mysql:

Keywords may be entered in any lettercase. The following queries are equivalent:

Here is another query. It demonstrates that you can use mysql as a simple calculator:

The queries shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:

A query need not be given all on a single line, so lengthy queries that require several lines are not a problem. mysql determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, mysql accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here is a simple multiple-line statement:

In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how mysql indicates that it has not yet seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you can always be aware of what mysql is waiting for.

If you decide you do not want to execute a query that you are in the process of entering, cancel it by typing \c:

Here, too, notice the prompt. It switches back to mysql> after you type \c, providing feedback to indicate that mysql is ready for a new query.

The following table shows each of the prompts you may see and summarizes what they mean about the state that mysql is in.

Multiple-line statements commonly occur by accident when you intend to issue a query on a single line, but forget the terminating semicolon. In this case, mysql waits for more input:

If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely mysql is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and mysql executes it:

The '> and "> prompts occur during string collection (another way of saying that MySQL is waiting for completion of a string). In MySQL, you can write strings surrounded by either ' or " characters (for example, 'hello' or "goodbye"), and mysql lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you have entered a line containing a string that begins with a ' or " quote character, but have not yet entered the matching quote that terminates the string. This often indicates that you have inadvertently left out a quote character. For example:

If you enter this SELECT statement, then press Enter and wait for the result, nothing happens. Instead of wondering why this query takes so long, notice the clue provided by the '> prompt. It tells you that mysql expects to see the rest of an unterminated string. (Do you see the error in the statement? The string 'Smith is missing the second single quotation mark.)

At this point, what do you do? The simplest thing is to cancel the query. However, you cannot just type \c in this case, because mysql interprets it as part of the string that it is collecting. Instead, enter the closing quote character (so mysql knows you've finished the string), then type \c:

The prompt changes back to mysql>, indicating that mysql is ready for a new query.

The `> prompt is similar to the '> and "> prompts, but indicates that you have begun but not completed a backtick-quoted identifier.

It is important to know what the '>, ">, and `> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type appear to be ignored by mysql—including a line containing QUIT. This can be quite confusing, especially if you do not know that you need to supply the terminating quote before you can cancel the current query.