dynamic sql in plsql - sql

Question

I have some code like this:
declare
p_vara varchar2;
p_varb varchar2;
p_varc varchar2;
begin
INSERT INTO MY_INSERTABLE_TABLE
SELECT a.id,b.id,c.id
FROM table_a a, table_b b, table_c c
WHERE a.id is not null
and a.id = b.id
and c.id = 'cat'
end;
Now based on the the variable to make it conditional so that only certain parts of the query get called based on the variable.
declare
p_vara varchar2;
p_varb varchar2;
p_varc varchar2;
begin
INSERT INTO MY_INSERTABLE_TABLE
SELECT a.id, -- Show only if p_vara = 'yes'
b.id, -- Show only if p_varb = 'yes'
c.id -- Show only if p_varc = 'yes'
FROM table_a a, -- Use only if p_vara = 'yes'
table_b b, -- Use only if p_varb = 'yes'
table_c c -- Use only if p_varc = 'yes'
WHERE a.id is not null -- Use only if p_vara = 'yes'
and a.id = b.id -- Use only if p_vara = 'yes' and p_varb = 'yes'
and c.id = 'cat' -- Use only if p_varc = 'yes'
end;
So for example if the variables are set as this:
p_vara = 'yes'
p_varb = 'no'
p_varc = 'no'
Then the query should look like this:
SELECT a.id
FROM table_a
WHERE a.id is null;

Answer 1

As written, your requirements do not appear to be complete. If all three variables are yes, for example, your full statement would have three table joins but only one join condition so you'd generate a Cartesian product with table_c. If p_vara = 'yes' and p_varc = 'yes', you'd have two tables joined with no join condition so you'd again have a Cartesian product. It seems unlikely to me that you really want to generate a Cartesian product.
In general, you can build a SQL statement in a string variable and then pass that to EXECUTE IMMEDIATE. If you have 25 boolean variables, that implies that your code could generate a total of 33.55 million distinct SQL statements. Just verifying that none of those paths generate a statement with syntax errors would be non-trivial. Combined with the fact that resorting to dynamic SQL generally makes code harder to read, maintain, and support in addition to creating opportunities for performance and security issues, I would tend to push back on any design that contemplates something as complex as what you are describing.
That said, you could do something like this (I'm not building the WHERE clause out completely but I trust you get the jist)
declare
l_vara boolean;
l_varb boolean;
l_varc boolean;
l_sql_stmt varchar(4000);
begin
l_sql_stmt := 'INSERT INTO my_insertable_table( col1, col2, col3 ) ';
l_sql_stmt := l_sql_stmt || ' SELECT ' ||
(case when l_vara then ' a.id, ' else ' null, ' end) ||
(case when l_varb then ' b.id, ' else ' null, ' end) ||
(case when l_varc then ' c.id, ' else ' null, ' end);
l_sql_stmt := rtrim( l_sql_stmt, ',' ); -- remove the extra trailing comma
l_sql_stmt := l_sql_stmt || ' FROM ';
if( l_vara )
then
l_sql_stmt := l_sql_stmt || ' table_a a, ';
end if;
if( l_varb )
then
l_sql_stmt := l_sql_stmt || ' table_b b, ';
end if;
if( l_varc )
then
l_sql_stmt := l_sql_stmt || ' table_c c, ';
end if;
-- again remove the extra trailing comma
l_sql_stmt := rtrim( l_sql_stmt, ',' );
<<build out the WHERE clause similarly>>
-- Log the SQL statement so you can debug it when it fails
insert into some_log_table( sql_stmt ) values( l_sql_stmt );
EXECUTE IMMEDIATE l_sql_stmt;
end;