动态选择哪个表

Question

I want to have the query dynamically choose which table it looks up against based on a value in a particular row in another table.

I have this query:

SELECT d.name

FROM `database1`.domains AS d
WHERE (SELECT COUNT(u.id) FROM <<d.db_name>>.users u) > 0

I want to use the value of d.db_name as database name. Example: d.db_name = database2

i want this:

SELECT d.name

FROM `database1`.domains AS d
WHERE (SELECT COUNT(u.id) FROM `database2`.users u) > 0

Answer 1

You could use variables for this:

SET @table_name = "some_table";

SELECT * FROM @table_name;

If you want to change the variable value depending on the results of your select you coudl use IF like this:

IF(some_column>50, @table_name := "value for true", @table_name := "value for false");