如何在 MySQL 查询中使用 IF ELSE 语句选择数据

Question

I get data from MySQL with if statement as code below. but I get an error: Error Code: 1241. Operand should contain 1 column(s) .

so can someone can help me?

I can not put it into the procedure, because I am using spring + mybatis in the project and i will put this code into them.

SELECT IF((SELECT COUNT(*) FROM table1 WHERE someField = 'A') < 0,
        (SELECT * FROM table2 WHERE someField = 'A'),
        (SELECT * FROM table1 WHERE someField = 'A'))

Answer 1

From the comments: I wanna get data of 1 in 2 tables when table 1 don't have data I will get data from table 2 .

That sounds like union all and not exists :

select * from table1 where someField = 'A'
union all
select * from table2 where not exists(select 1 from tabe1 where someField = 'A'

Note that, for this to work, both tables must contain exactly the same number of columns, whith aligned datatypes. You should really be enumerating the columns that you want to show in the resultset in both unioned queries, to avoid any possible ambiguity. If needed, you can cast columns or add litteral values to any or both of the resultset to align the resultsets (without seeing your actual data structures, I cannot tell how to do).