Mysql How to only select from a column if the column exists

问题

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.

回答1:

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.

回答2:

I came across the same situation where I had some product tables created by sheets uploaded by users. Sometimes, the sheets did not have column named "obsolete", so I had to import all products from the sheet but not the obsolete ones.

I am not modifying my query based on the original question that was asked, but here is my solution:

SELECT
    t2.model,
    (
        SELECT
            COUNT(*) AS _count
        FROM db.table t1
        WHERE
            `obsolete`=1
            AND t1.model=t2.model
    ) AS `obsolete`
FROM (
    SELECT
        0 AS `obsolete`,
        t3.model
    FROM db.table t3
) t2

There are 2 most important parts in this query:

We are selecting 0 AS obsolete as dummy to fool MySql which will be used even if column does not exist when selecting COUNT(*).
We have named tables as t1 & t2 to match the column model as t1.model=t2.model.

来源：https://stackoverflow.com/questions/24194223/mysql-how-to-only-select-from-a-column-if-the-column-exists

标签

mysql

select

if-statement

exists