Entering
and Executing Commands on MySQL Client
This section describes the basic principles of entering
commands, using several queries you can try out to familiarize yourself with
how mysql works. Here is a simple command 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:
mysql>
SELECT
VERSION(), CURRENT_DATE;
+--------------+--------------+
|
VERSION() | CURRENT_DATE |
+--------------+--------------+
|
5.5.0-m2-log | 2009-05-04 |
+--------------+--------------+
1
row in set (0.01 sec)
mysql>
·
When you issue a
command, mysql sends it to the server for execution
and displays the results, then prints another
mysql> prompt to indicate that it is ready for another command.
·
mysql displays query output in tabular form
(rows and columns). The first row contains labels for the columns. The rows
following are the query results. Normally, column labels are the names of the
columns you fetch from database tables. If you're retrieving the value of an
expression rather than a table column (as in the example just shown), mysql labels the column using the expression
itself.
·
mysql shows how many rows were returned and
how long the query took to execute, which gives you a rough idea of server
performance. These values are imprecise because they represent wall clock time
(not CPU or machine time), and because they are affected by factors such as
server load and network latency.
Keywords may be entered in any lettercase. The following
queries are equivalent:
mysql>SELECT VERSION(), CURRENT_DATE;
mysql>select version(), current_date;
mysql>SeLeCt vErSiOn(), current_DATE;
Here is another query. It
demonstrates that you can use mysql as a simple calculator:
mysql>SELECT SIN(PI()/4), (4+1)*5;
+------------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+------------------+---------+
| 0.70710678118655 | 25 |
+------------------+---------+
1 row in set (0.02 sec)
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:
mysql>SELECT VERSION(); SELECT NOW();
+--------------+
| VERSION() |
+--------------+
| 5.5.0-m2-log |
+--------------+
1 row in set (0.00 sec)
+---------------------+
| NOW() |
+---------------------+
| 2009-05-04 15:15:00 |
+---------------------+
1 row in set (0.00 sec)
A command
need not be given all on a single line, so lengthy commands 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. Here is a simple
multiple-line statement:
mysql>SELECT
->USER()
->,
->CURRENT_DATE;
+---------------+--------------+
| USER() | CURRENT_DATE |
+---------------+--------------+
| jon@localhost | 2005-10-11 |
+---------------+--------------+
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.
If you
decide you do not want to execute a command that you are in the process of
entering, cancel it by typing
\c:mysql>SELECT
->USER()
->\c
mysql>
Here, too, notice
the prompt. It switches back to
mysql> after you type \c,
providing feedback to indicate thatmysql is ready for a new command.
Multiple-line
statements commonly occur by accident when you intend to issue a command on a
single line, but forget the terminating semicolon. In this case, mysql waits for more input:
mysql>SELECT USER()
->
If this happens to
you (you think you've entered a statement but the only response is a
-> prompt), most likelymysql 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:mysql>SELECT USER()
->;
+---------------+
| USER() |
+---------------+
| jon@localhost |
+---------------+
