Mysql如果列存在，如何仅从列中进行选择 [英] Mysql How to only select from a column if the column exists

问题描述

我需要能够检查列是否存在，如果它存在，那么我想从中进行SELECT。

I need to be able to check if a column exists and if it does then I want to SELECT from it.

我正在尝试很多不同的变体但是我我甚至不确定这是否可能。

I am trying lots of different variations but I'm not even sure if this is possible.

这是我的最新尝试：

SELECT
IF (EXISTS (SELECT `Period` AS `Period` FROM myview), `PERIOD`,
IF (EXISTS (SELECT `Country` AS `COUNTRY` FROM myview),`COUNTRY` FROM myview ;

有任何想法吗？

编辑

我在这里看到了另一个问题：< a href =https://stackoverflow.com/questions/3395798/mysql-check-if-a-column-exists-in-a-table-with-sql> MySQL，检查表中是否存在列SQL

I had seen the other question on here: MySQL, Check if a column exists in a table with SQL

但我仍然在努力使用if语句。我可以使用上面问题中的答案检查列是否存在。但我的问题是 - 如果发现结果为真，如何从该列执行select语句。

But I still struggle with the if statement. I can check to see if the column exists using the answer in the question above. But my question is - how to execute a select statement from that column if the result is found to be true.

编辑2

下面的答案表明我应该使用BEGIN和END语句，这是有道理的。但是，我的查询在第一行抱怨。它说'意外的IF' - 任何人都可以确认这是否是MYSQL的正确语法？

The answer below indicates that I should use the BEGIN and END statement and this makes sense. However, my query complains at the first line. It says 'unexpected IF' - can anybody confirm if this is the right syntax fro MYSQL?

if( exists (SELECT * 
    FROM information_schema.COLUMNS 
    WHERE TABLE_SCHEMA = 'db_name' 
    AND TABLE_NAME = 'view_name' 
    AND COLUMN_NAME = 'column_name') )
begin
    SELECT `column_name` FROM `view_name`
end

提前致谢。

推荐答案

此查询将显示列是否存在。

This query will give you whether a column exists.

SELECT * 
FROM information_schema.COLUMNS 
WHERE 
    TABLE_SCHEMA = 'db_name' 
AND TABLE_NAME = 'table_name' 
AND COLUMN_NAME = 'column_name'

如果你想要要检查是否存在某些列然后执行select语句，您需要首先检查列是否存在。然后执行select：

If you want to check if some columns exist then perform a select statement you need to first check your columns exist. Then perform the select:

if (exists (SELECT * FROM information_schema.COLUMNS WHERE TABLE_NAME = 'myview' AND COLUMN_NAME = 'Period') and exists (SELECT * FROM information_schema.COLUMNS WHERE TABLE_NAME = 'myview' AND COLUMN_NAME = 'Country'))
begin
    select `Period`, `Country` from myview
end

如果IF条件为真，那么您将执行BEGIN和END内的任何内容。

If the IF condition is true, then you will execute anything inside the BEGIN and END.

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