MySQL之SELECT

下面是連接MySQL后，獲取的SELECT 語句的幫助：

mysql> HELP SELECT
Name: 'SELECT
Description://描述
Syntax://語法
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

接下來我們一起看一下，SELECT 語句的一些參數和用法：
SELECT ->本意: 即為 選擇、挑選；

[ALL | DISTINCT | DISTINCTROW ]

All：所有（默認）；
DISTINCT：獨特的，顯示不同的；
DISTINCTROW：顯示不同的行；
mysql> SELECT [DISTINCT/DISTINCTROW] column_name FROM table_name;
ALL參數為默認的參數，而DISTINCT/DISTINCTROW為可選的參數，上面的語句執行后，即可顯示table_name表的column_name 中不同的元素，如果把column_name 改成 * 即為選擇全部，而再加上，這些參數，意思即為選擇不一樣的行，而且在這里DISTINCT/DISTINCTROW沒有區別。

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