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

我需要能够检查列是否存在，如果存在，那么我想从中进行SELECT。

我正在尝试很多不同的变化，但我甚至不确定这是否可行。

这是我最近的尝试：

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： MySQL，检查SQL中是否存在列

但我仍然对if语句感到困惑。 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。

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？

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。

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

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

Here's my latest attempt:

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

Any ideas?

EDIT

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

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.

EDIT 2

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

Thanks in advance.

最满意答案

此查询将为您提供列是否存在。

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

如果要检查是否存在某些列，则执行select语句，首先需要检查列是否存在。 然后执行选择：

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。

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'

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

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

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

我需要能够检查列是否存在，如果存在，那么我想从中进行SELECT。

我正在尝试很多不同的变化，但我甚至不确定这是否可行。

这是我最近的尝试：

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： MySQL，检查SQL中是否存在列

但我仍然对if语句感到困惑。 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。

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？

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。

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

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

Here's my latest attempt:

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

Any ideas?

EDIT

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

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.

EDIT 2

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

Thanks in advance.

最满意答案

此查询将为您提供列是否存在。

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

如果要检查是否存在某些列，则执行select语句，首先需要检查列是否存在。 然后执行选择：

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。

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'

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

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