Question

我有一个应该执行示例代码的存储过程：

＆＃13;

IF EXISTS (
               SELECT TABLE_NAME
               FROM INFORMATION_SCHEMA.COLUMNS
               WHERE TABLE_NAME IN ('TB1', 'TB2', 'TB3')
               AND COLUMN_NAME = 'COL'
              AND table_schema = 'TestDB' 
                  GROUP BY TABLE_NAME
                  HAVING count (TABLE_NAME) = 3
               )  
                THEN
                   /*Used for debugging*/ 
                    SELECT 'Tables EXIST' INTO msg;  
                ELSE
                   /*Used for debugging*/   
                    SELECT 'NON-EXISTENT TABLES !' INTO msg; 
                END IF

＆＃13;

基本上，我希望上面的代码段选择所有具有名为＆＃39; COL＆＃39;的列的指定表。我只想使用information_schema来做这件事，没有别的。

检查后，测试语句（IF EXISTS）始终返回false。也就是说，即使我的三个测试表（TB1，TB2和TB3）具有COL字段，ELSE语句也始终执行。我犯了什么罪？

Answer 1

与此平行的东西应该有效

select distinct 
    table_name 
from information_schema.columns
    where column_name = 'CHARACTER_SET_NAME'
    and table_name in ('CHARACTER_SETS', 'COLLATIONS', 'COLUMNS', 'TRIGGERS', 'FOO', 'BAR')
;

你有这个

       SELECT TABLE_NAME
       FROM INFORMATION_SCHEMA.COLUMNS
       WHERE TABLE_NAME IN ('TB1', 'TB2', 'TB3')
       AND COLUMN_NAME = 'COL'
       AND table_schema = 'TestDB' 
          GROUP BY TABLE_NAME
          HAVING count (TABLE_NAME) = 3
       )

即。这样：

select distinct
    table_name
from information.schema.columns
    where table_name in ('TB1', 'TB2', 'TB3')
    and column_name = 'COL'
    and table_schema = 'TestDB'

如果你想确保所有三个表都存在并且有一个＆＃39; COL＆＃39;列写一个条件来检查这个值是否为3

select count(*) from (
select distinct
    table_name
from information.schema.columns
    where table_name in ('TB1', 'TB2', 'TB3')
    and column_name = 'COL'
    and table_schema = 'TestDB'
) tbls
;

Answer 2

你可以这样做：

功能：

CREATE DEFINER=`root`@`localhost` FUNCTION `new_function`() RETURNS varchar(64) CHARSET utf8
BEGIN
    declare cnt int;

    declare msg varchar(64);

    select count(*) from (
    select distinct
        table_name
    from information_schema.columns
        where table_name in ('TB1', 'TB2', 'TB3')
        and column_name = 'COL'
        and table_schema = 'TestDB'
    ) tbls into cnt;

    if cnt = 3 then
        select 'tables exist' into msg;
    else
        select 'tables do not exists' into msg;
    end if;

RETURN msg;
END

然后运行：

create schema TestDB;

use TestDB;

create table TB1(
    COL int
);

create table TB2(
    COL int
);

create table TB3(
    COL int
);    

select new_function();

这将返回：表存在

计算具有相似列名的所有MySQL表

2 个答案: