MySQL if / else or select if(contiton,'true','false'); - 语法问题

Question

When I use

select if (1=1,'true','false');

then everything is ok and I receive 'true' but if I try

select if (1=1,(select * from table1), (select * from table2));`

I receive syntax error in mysql! With

if condition then <SQL-Expression1> else <SQL-Expression2> end if;

I have the same problem, when the SQL-Expression is complex like

select * from table1!

If I use a complex SQL-Expression like

select * from table1 

or

insert into table1 (select field1 from table2 where 
                    field1>(select Max(field) from table1));

then I always receive a syntax error, when such a expression is included in an if/else-Statement!

How can I fit it, that complex sql-Statements can be choosed?

My problem is:

I made 2 tables like

create table1 (x1 int4, x2 int4, x3 int4);
create table2 (x int4);
insert into table1 values(1,2,3);
insert into table1 values(4,0,5);

I wanted to transponse table1 to table2

For example:

The result should in table2 like this

1
2
3
4
5

If I enlarge table 1 with a new line like

insert into table1 values (6,7,8);

then table2 should be changed to

1
2
3
4
5
6
7
8

I tried to make it in this way

select if ((select count(*) from table2)=0,
          (insert into table2 (select x1 from table1 where x1>0)),
          (insert into table2 (select x1 from table1 where 
                               x1>(select Max(x) from table1))));

The same also with x2 and x3. but syntax errors occur!

If I use only

insert into table2 (select x1 from table1 where x1>(select Max(x) from table1));

then it works if table1 ist not empty otherwise I had to do

insert into table2 (select x1 from table1 where x1>0);

Answer 1

Subqueries in the SELECT clause may only return single values; ie not more than one result, and not more than one column. You can't "trick" a query into returning a varying number of columns.

Also, the if-then form of "if" is reserved for procedural sql; used in stored procedures, triggers, etc...