存储过程
****************************************

创建存储过程
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.. code-block:: mysql

    create
        # 创建存储过程: https://dev.mysql.com/doc/refman/8.0/en/create-procedure.html
        definer = `mysql.sys`@localhost procedure table_exists(IN in_db varchar(64), IN in_table varchar(64),
                                                               OUT out_exists enum ('', 'BASE TABLE', 'VIEW', 'TEMPORARY'))
        # 存储过程的注释
        comment '
    笔者注
    -----

    摘自内置存储过程: sys.table_exists

    Description
    -----------

    Tests whether the table specified in in_db and in_table exists either as a regular
    table, or as a temporary table. The returned value corresponds to the table that
    will be used, so if there''s both a temporary and a permanent table with the given
    name, then ''TEMPORARY'' will be returned.

    Parameters
    -----------

    in_db (VARCHAR(64)):
      The database name to check for the existance of the table in.

    in_table (VARCHAR(64)):
      The name of the table to check the existance of.

    out_exists ENUM('''', ''BASE TABLE'', ''VIEW'', ''TEMPORARY''):
      The return value: whether the table exists. The value is one of:
        * ''''           - the table does not exist neither as a base table, view, nor temporary table.
        * ''BASE TABLE'' - the table name exists as a permanent base table table.
        * ''VIEW''       - the table name exists as a view.
        * ''TEMPORARY''  - the table name exists as a temporary table.

    Example
    --------

    mysql> CREATE DATABASE db1;
    Query OK, 1 row affected (0.07 sec)

    mysql> use db1;
    Database changed
    mysql> CREATE TABLE t1 (id INT PRIMARY KEY);
    Query OK, 0 rows affected (0.08 sec)

    mysql> CREATE TABLE t2 (id INT PRIMARY KEY);
    Query OK, 0 rows affected (0.08 sec)
    '
        # 访问控制 invoker或definer，决定调用存储过程的方式
        # 文档: https://dev.mysql.com/doc/refman/8.0/en/stored-objects-security.html
        # 博客: https://www.cnblogs.com/gaoyuechen/p/9407463.html
        sql security invoker
    BEGIN
        # 实现具体业务逻辑的代码
    END;

编译sql和使用占位符变量
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https://dev.mysql.com/doc/refman/8.0/en/sql-prepared-statements.html