Oracle：如何从动态生成的查询中获取最大列长？

Question

Suppose I have a query like

select a.column1, b.column2, substr(b.column3,instr(b.column3,',')) as Column3
from tableA a, tableB b
where a.column1 = b.column1

How do I get the max character length for each column of this query ?

If tableA and tableB are defined as

tableA ( column1 VARCAHR2(100))     
tableB ( column1 VARCAHR2(100), column2 INTEGER, column3 VARCAHR2(5000))

I would like to get those data :

COLUMN1 : 100
COLUMN2 : 10
COLUMN3 : 5000

Keep in mind this is a dynamic query, there might be any number of columns from any number of tables within limitation.

Answer 1

You can create a table for a moment from your query (CTAS) and after that use user_tab_columns to get column's data_length for your new table.

create table temp as 
select a.column1, b.column2, substr(b.column3,instr(b.column3,',')) as Column3
from tableA a, tableB b
where a.column1 = b.column1
    --No need for store the data, unless you want the maximum size of current data.
    and 1 = 0;

select column_name, data_length from user_tab_columns where table_name = 'TEMP';

drop table temp;

It's quite easy and you do not need to care what you have exactly in your query. Oracle will figure out this for you.

BTW. The max size of VARCHAR2 is 4000. You cannot have 5000 unless you are using 12c with extended data types.

Answer 2

You might have to access the USER_TAB_COLUMNS metadata table to get these column lengths

Below is an example query

SELECT COLUMN_NAME
      ,CHAR_LENGTH
  FROM USER_TAB_COLUMNS 
 WHERE TABLE_NAME IN ('tableA', 'tableB')
   AND COLUMN_NAME IN ('column1', 'column2', 'column3')
;

If you need the above in your generated query, then it would need to change to something like below

select a.column1
     , b.column2
     , substr(b.column3,instr(b.column3,',')) as Column3
     , (SELECT CHAR_LENGTH FROM USER_TAB_COLUMNS WHERE TABLE_NAME = 'tableA' AND COLUMN_NAME = 'column1') AS column1_size
     , (SELECT CHAR_LENGTH FROM USER_TAB_COLUMNS WHERE TABLE_NAME = 'tableB' AND COLUMN_NAME = 'column2') AS column2_size
     , (SELECT CHAR_LENGTH FROM USER_TAB_COLUMNS WHERE TABLE_NAME = 'tableB' AND COLUMN_NAME = 'column3') AS column3_size
  from tableA a
     , tableB b
 where a.column1 = b.column1

The above is one way of doing this just as an example. Depending on what you are trying to do and how you are generating the query, I think you would want to get the column sizes in a separate query like the first one, since you already know the column and table names when generating the query.

Hope this helps..