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3.2 Entering Queries

3.2输入查询

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.

确保您已连接到云服务器，如上一节所述。这样做本身并不会选择任何数据库（选择数据库需要use database 命令）来处理，但是没关系。这一节，更重要的是如何进行查询，而不是创建表，插入数据，检索数据。本节介绍输入查询的基本原理，使用多个查询可以让您熟悉mysql的 工作原理。

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:

这是一个简单的查询，要求云服务器告诉您其版本号和当前日期。在mysql> 提示符后输入并按回车键。

mysql> SELECT VERSION(), CURRENT_DATE;
+--------------+--------------+
| VERSION()    | CURRENT_DATE |
+--------------+--------------+
| 5.6.1-m4-log | 2010-08-06   |
+--------------+--------------+
1 row in set (0.01 sec)
mysql>

This query illustrates several things about mysql:

这个查询说明了几个关于mysql的事情 ：

• A query 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.)

  一个查询之后通常跟随一个分号（有一些例外，可以省略一个分号。QUIT是其中之一，稍后我们会接触到其他可以忽略分号的语句）

• When you issue a query, 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 query.

  当您发出查询时，mysql将其发送到云服务器执行并显示结果，然后打印另一个mysql>提示，指示它已准备好进行另一个查询。

• 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), mysqllabels the column using the expression itself.

  mysql以表格形式（行和列）显示查询输出。第一行是列名，下面的行是查询结果。通常，列名是从数据库表中获取的列的名称，如果您使用表达式的值而不是表列（如示例），则 mysql将使用表达式本身对该列进行标记。

• 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 sometimes not shown in the remaining examples in this chapter.)

  mysql显示了返回的行数以及查询执行了多长时间，这让您粗略的了解云服务器的性能。这些值不精确，因为它们代表系统时间（不是CPU或机器时间），并且因为它们受到诸如云服务器负载和网络延迟等因素的影响。（为了简便起见，“row in set ” 行 有时不在本章剩余的案例中展现）。

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

关键字不区分大小写。以下查询是等效的：

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:

这是另一个查询。这表明你可以使用 mysql作为一个简单的计算器：

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:

迄今为止所显示的查询是相对较短的单行语句。您甚至可以在一行中输入多个语句。只需用分号结束每一个：

mysql> SELECT VERSION(); SELECT NOW();
+--------------+
| VERSION()    |
+--------------+
| 5.6.1-m4-log |
+--------------+
1 row in set (0.00 sec)
+---------------------+
| NOW()               |
+---------------------+
| 2010-08-06 12:17:13 |
+---------------------+
1 row in set (0.00 sec)

A query need not be given all on a single line, so lengthy queries that require several lines are not a problem. mysqldetermines 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:

mysql> SELECT    -> USER()    -> ,    -> CURRENT_DATE;+---------------+--------------+| USER()        | CURRENT_DATE |+---------------+--------------+| jon@localhost | 2010-08-06   |+---------------+--------------+

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:

mysql> SELECT    -> USER()    -> \cmysql>

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.

Prompt	Meaning
mysql>	Ready for new query
->	Waiting for next line of multiple-line query
'>	Waiting for next line, waiting for completion of a string that began with a single quote (')
">	Waiting for next line, waiting for completion of a string that began with a double quote (")
`>	Waiting for next line, waiting for completion of an identifier that began with a backtick (`)
/*>	Waiting for next line, waiting for completion of a comment that began with /*

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:

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 executes it:

mysql> SELECT USER()    -> ;+---------------+| USER()        |+---------------+| jon@localhost |+---------------+

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"), andmysql 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:

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 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 mysqlknows you've finished the string), then type \c:

mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
    '> '\cmysql>

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.

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