I have a attribute with the data type char(256). I import the value via SQL Developer from a csv file
When the attribute gets a value with 10 characters, the remaining space gets filled with spaces.
I know that char allocates the space staticly, but does that also mean that I get a string in the format like "abc " ?
Since this make sql statements with equal operators difficult.
You are operating under a misconception; it has nothing to do with SQL Developer.
A CHAR data-type is a fixed-length string; if you do not provide a string of the full length then Oracle will right-pad the string with space (ASCII 32) characters until it has the correct length.
From the documentation:
CHAR Datatype
The CHAR datatype stores fixed-length character strings. When you create a table with a CHAR column, you must specify a string length (in bytes or characters) between 1 and 2000 bytes for the CHAR column width. The default is 1 byte. Oracle then guarantees that:
When you insert or update a row in the table, the value for the CHAR column has the fixed length.
If you give a shorter value, then the value is blank-padded to the fixed length.
If a value is too large, Oracle Database returns an error.
Oracle Database compares CHAR values using blank-padded comparison semantics.
To solve this, do not use CHAR for variable length strings and use VARCHAR2 instead.
VARCHAR2 and VARCHAR Datatypes
The VARCHAR2 datatype stores variable-length character strings. When you create a table with a VARCHAR2 column, you specify a maximum string length (in bytes or characters) between 1 and 4000 bytes for the VARCHAR2 column. For each row, Oracle Database stores each value in the column as a variable-length field unless a value exceeds the column's maximum length, in which case Oracle Database returns an error. Using VARCHAR2 and VARCHAR saves on space used by the table.
You may use varchar2 instead of char as datatype to avoid this.
Or you can trim your data in query by using rtrim(columnname) .
Related
I am working with an Oracle DB 11g
I have a database table with the primary key being a CHAR(4) - Though only numbers are used for this column.
I noticed that there are some records that for example show '0018' or '0123'.
So few things I noticed odd and needed some help on
-Does a CHAR column "automatically" pad zeros to a value?
-Also I noticed when writing a SQL that if I DONT use quotes in my where clause that it returns results, but if I do use quotes it does not? So for example
DB CHAR(4) column has a key of '0018'
I use this query
SELECT * FROM TABLE_A WHERE COLUMN_1=18;
I get the row as expected.
But when I try the following
SELECT * FROM TABLE_A WHERE COLUMN_1='18';
This does NOT work but this does work again
SELECT * FROM TABLE_A WHERE COLUMN_1='0018';
So I am a bit confused how the first query can work as expected without quotes?
Does a CHAR column "automatically" pad zeros to a value?
No. From the documentation:
If you insert a value that is shorter than the column length, then Oracle blank-pads the value to column length.
So if you insert the number 18 it will be implicitly converted to the string '18 ', with two trailing spaces. You can see that in this fiddle, which also shows the comparisons.
That means something else is zero-padding your data - either your application/code before inserting, or possibly in a trigger.
Also I noticed when writing a SQL that if I DONT use quotes in my where clause that it returns results, but if I do use quotes it does not
The data type comparison and conversion rules are shown in the documentation too:
When comparing a character value with a numeric value, Oracle converts the character data to a numeric value.
When you do:
SELECT * FROM TABLE_A WHERE COLUMN_1=18;
the string '0018' is implicitly converted to the number 18 so that it can be compared with your numeric literal. The leading zeros are meaningless once it's converted, so '0018', '018 ' and 18 ' would all match.
With your zero-padded column value that matches and you do get a result: 18 ('0018' converted to a number) = 18
That means that every value in the table has to be converted before it can be compared; which also means that if you has a normal index on column_1 then it wouldn't be utilised in that comparison.
When you do:
SELECT * FROM TABLE_A WHERE COLUMN_1='18';
the column and literal are the same data type so no conversion has to be applied (so a normal index can be used). Oracle will use blank-padded comparison semantics here, because the column is char, padding the shorter literal value to the column size as '18 ', and then it will only match if the strings match exactly - so '18 ' would match but '0018' or ' 18 ' or anything else would not.
With your zero-padded column value that does not match and you don't get a result: '0018' != '18 ' ('18' padded to length 4)
When you do:
SELECT * FROM TABLE_A WHERE COLUMN_1='0018';
the column and literal are the same data type so no conversion, no padding is applied as the literal is already the same length as the column value, and again it will only match if the strings match exactly - so '0018' would match but '18 ' or ' 18 ' or anything else would not.
With your zero-padded column value that matches and you do get a result: '0018' = '0018'
Does a CHAR column "automatically" pad zeros to a value?
Not always zero's sometimes spaces. if all characters values are numeric yes it will pad zeros up to a fixed size of the character field.
So I am a bit confused how the first query can work as expected without quotes?
Because of implicit type conversions. The system is casting either the char to numeric or the numeric to char in which case it either drops the leading zeros and compares numeric values or it pads to be of the same data type and then compares. I'm pretty sure it's going character to numeric and thus the leading zeros are dropped when comparing.
See: https://docs.oracle.com/cd/B13789_01/server.101/b10759/sql_elements002.htm for more details on data type comparison and implicit casting
More:
in the case of : SELECT * FROM TABLE_A WHERE COLUMN_1='18'; I
think the 18 is already a character data so it becomes '18 ' (note 2 spaces after 18)
compared to '0018'
SELECT * FROM TABLE_A WHERE COLUMN_1=18; columN_1 gets cast to numeric so 18=18
SELECT * FROM TABLE_A WHERE COLUMN_1='0018'; column_1 is already a char(4) so '0018' = '0018'
What is the difference between CHAR() and VARCHAR() declarations from HQL?
VARCHAR holds the advantage since variable-length data would produce smaller rows and, thus, smaller physical files.
CHAR fields require less string manipulation because of fixed field widths. Partiton, lookup, join, group on CHAR field are faster than VARCHAR fields.
like in any other language:
CHAR is fixed length character datatype , for example If you define char(10) and the input value is of 6 characters then the remaining 4 will be filled with spaces.
VARCHAR has variable length, for example If you define varchar(10) and the input value is of 6 characters then only 6 bytes will be used and no additional space will be blocked.
HIVE DOC REFERENCE
I have some large varchar values in Postgres that I want to SELECT and move somewhere else. The place they are going to uses VARCHAR(4095) so I only need at most 4095 bytes (I think that's bytes) and some of these varchars are quite big, so a performance optimization would be to SELECT a truncated version of them.
How can I do that?
Something like:
SELECT TRUNCATED(my_val, 4095) ...
I don't think it's a character length though, it needs to be a byte length?
The n in varchar(n) is the number of characters, not bytes. The manual:
SQL defines two primary character types: character varying(n) and
character(n), where n is a positive integer. Both of these types can
store strings up to n characters (not bytes) in length.
Bold emphasis mine.
The simplest way to "truncate" a string would be with left():
SELECT left(my_val, 4095)
Or just cast:
SELECT my_val::varchar(4095)
The manual once more:
If one explicitly casts a value to character varying(n) or
character(n), then an over-length value will be truncated to n
characters without raising an error. (This too is required by the SQL standard.)
I am using MySQL data base with Rails. I have created a field of type string. Are there any limits to its length? What about type text?
Also as text is variable sized, I believe there would be extra costs associated with using text objects. How important can they get, if at all?
CHAR
A fixed-length string that is always right-padded with spaces to the specified length when stored The range of Length is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. CHAR values are sorted and compared in case-insensitive fashion according to the default character set unless the BINARY keyword is given.
VARCHAR
A variable-length string. Note: Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification)
The range of Length is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given
TINYBLOB, TINYTEXT
A TINYBLOB or TINYTEXT column with a maximum length of 255 (28 - 1) characters
BLOB, TEXT
A BLOB or TEXT column with a maximum length of 65,535 (216 - 1) characters , bytes = 64 KiB
MEDIUMBLOB, MEDIUMTEXT
A MEDIUMBLOB or MEDIUMTEXT column with a maximum length of 16,777,215 (224 - 1)characters , bytes = 16 MiB
LONGBLOB, LONGTEXT
A LONGBLOB or LONGTEXT column with a maximum length of 4,294,967,295 (232 - 1) characters , bytes = 4 GiB
See MySQL Data Types Quick Reference Table for more info.
also you can see MYSQL - String Type Overview
String, in general, should be used for short text. For example, it is a VARCHAR(255) under MySQL.
Text uses the larger text from the database, like, in MySQL, the type TEXT.
For information on how this works and the internals in MySQL and limits and such, see the other answer by Pekka.
If you are requesting, say, a paragraph, I would use text. If you are requesting a username or email, use string.
See the mySQL manual on String Types.
Varchar (String):
Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions. The effective maximum length of a VARCHAR in MySQL 5.0.3 and later is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.
Text: See storage requirements
If you want a fixed size text field, use CHAR which can be 255 characters in length maximum. VARCHAR and TEXT both have variable length.
I am loading and inserting data into an Oracle database. When I encounter special characters that look like Chinese characters, I am getting an error like row rejected because maximum size of column was exceeded. I am not getting this error for rows which have English characters which appear to be of same length for same column. I am using SUBSTR and TRIM function but it is not working. How can I determine whether the length of a string which is in Chinese exceeds column size?
if your columns are defined as VARCHAR2(XX) [for example VARCHAR2(20)], you will receive an error if you try to insert a string that is more than XX bytes long.
The function SUBSTR calculates length in number of characters, not bytes. To select a substring in bytes, use the function SUBSTRB.
SQL> select substr('ЙЖ', 1, 2) from dual;
SUBSTR('ЙЖ',1,2)
------------------
ЙЖ
SQL> select substrb('ЙЖ', 1, 2) from dual;
SUBSTRB('ЙЖ',1,2)
-------------------
Й
Edit: As suggested by Adam, you can use character arithmetics if you define your columns and variables as VARCHAR2 (XX CHAR). In that case your columns will be able to store XX characters, in all character sets (up to a maximum of 4000 bytes if you store it in a table).