MySQL之SELECT

下面是连接MySQL后，获取的SELECT 语句的帮助：

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mysql> HELP SELECT
Name: 'SELECT
Description://描述
Syntax://语法
SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
        export_options
      | INTO DUMPFILE 'file_name'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]
SELECT is used to retrieve rows selected from one or more tables, and
can include UNION statements and subqueries. See [HELP UNION], and
http://dev.mysql.com/doc/refman/5.5/en/subqueries.html.
The most commonly used clauses of SELECT statements are these:
o Each select_expr indicates a column that you want to retrieve. There
  must be at least one select_expr.
o table_references indicates the table or tables from which to retrieve
  rows. Its syntax is described in [HELP JOIN].
o The WHERE clause, if given, indicates the condition or conditions
  that rows must satisfy to be selected. where_condition is an
  expression that evaluates to true for each row to be selected. The
  statement selects all rows if there is no WHERE clause.
  In the WHERE expression, you can use any of the functions and
  operators that MySQL supports, except for aggregate (summary)
  functions. See
  http://dev.mysql.com/doc/refman/5.5/en/expressions.html, and
  http://dev.mysql.com/doc/refman/5.5/en/functions.html.
SELECT can also be used to retrieve rows computed without reference to
any table.
URL: http://dev.mysql.com/doc/refman/5.5/en/select.html

接下来我们一起看一下，SELECT 语句的一些参数和用法：
SELECT ->本意: 即为 选择、挑选；

[ALL | DISTINCT | DISTINCTROW ]

All：所有（默认）；
DISTINCT：独特的，显示不同的；
DISTINCTROW：显示不同的行；
mysql> SELECT [DISTINCT/DISTINCTROW] column_name FROM table_name;
ALL参数为默认的参数，而DISTINCT/DISTINCTROW为可选的参数，上面的语句执行后，即可显示table_name表的column_name 中不同的元素，如果把column_name 改成 * 即为选择全部，而再加上，这些参数，意思即为选择不一样的行，而且在这里DISTINCT/DISTINCTROW没有区别。

  [HIGH_PRIORITY]
  [STRAIGHT_JOIN]
  [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
  [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]