( Entering queries

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 3.2 Entering Queries
 Make sure that you are connected to the server, as discussed in the
 previous section.  Doing so will not in itself select any database to
 work with, but that's okay.  At this point, it's 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 commands, using
 several queries you can try out to familiarize yourself with how
 `mysql' works.
 Here's 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:
      | VERSION()    | CURRENT_DATE |
      | 3.22.20a-log | 1999-03-19   |
      1 row in set (0.01 sec)
 This query illustrates several things about `mysql':
    * A command normally consists of an SQL statement followed by a
      semicolon.  (There are some exceptions where a semicolon may be
      omitted.  `QUIT', mentioned earlier, is one of them.  We'll get to
      others later.)
    * 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.  (For
      brevity, the "rows in set" line is not shown in the remaining
      examples in this chapter.)
 Keywords may be entered in any lettercase.  The following queries are
      mysql> select version(), current_date;
      mysql> SeLeCt vErSiOn(), current_DATE;
 Here's 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 |
      |    0.707107 |      25 |
 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()    |
      | 3.22.20a-log |
      | NOW()               |
      | 1999-03-19 00:15:33 |
 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.  (In other words, `mysql'
 accepts free-format input:  it collects input lines but does not
 execute them until it sees the semicolon.)
 Here's a simple multiple-line statement:
      mysql> SELECT
          -> USER()
          -> ,
          -> CURRENT_DATE;
      | USER()             | CURRENT_DATE |
      | joesmith@localhost | 1999-03-18   |
 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 hasn't 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 will always be aware
 of what `mysql' is waiting for.
 If you decide you don't want to execute a command that you are in the
 process of entering, cancel it by typing `\c':
      mysql> SELECT
          -> USER()
          -> \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 command.
 The following table shows each of the prompts you may see and
 summarizes what they mean about the state that `mysql' is in:
 `mysql>'Ready for new command.
 `       Waiting for next line of multiple-line command.
 `       Waiting for next line, collecting a string that begins
 '>'     with a single quote (`'').
 `       Waiting for next line, collecting a string that begins
 ">'     with a double quote (`"').
 `       Waiting for next line, collecting an identifier that
 `>'     begins with a backtick (``').
 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 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' will execute
      mysql> SELECT USER()
          -> ;
      | USER()             |
      | joesmith@localhost |
 The `'>' and `">' prompts occur during string collection.  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've 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.  That's fine if you really are entering a
 multiple-line string, but how likely is that?  Not very.  More often,
 the `'>' and `">' prompts indicate that you've inadvertently left out a
 quote character.  For example:
      mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
 If you enter this `SELECT' statement, then press Enter and wait for the
 result, nothing will happen.  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 quote.)
 At this point, what do you do?  The simplest thing is to cancel the
 command.  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':
      mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
          '> '\c
 The prompt changes back to `mysql>', indicating that `mysql' is ready
 for a new command.
 The ``>' prompt is similar to th `'>' and `">' prompts, but indicates
 that you have begun but not completed a backtick-quoted identifier.
 It's important to know what the `'>', `">', and ``>' prompts signify,
 because if you mistakenly enter an unterminated string, any further
 lines you type will appear to be ignored by `mysql'--including a line
 containing `QUIT'!  This can be quite confusing, especially if you
 don't know that you need to supply the terminating quote before you can
 cancel the current command.
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