MariaDB has many built-in functions that can be used to manipulate strings of data. With these functions, one can format data, extract certain characters, or use search expressions. Good developers should be aware of the string functions that are available. Therefore, in this article we will go through several string functions, grouping them by similar features, and provide examples of how they might be used.
There are several string functions that are used to format text and numbers for nicer display. A popular and very useful function for pasting together the contents of data fields with text is the CONCAT() function. As an example, suppose that a table called contacts has a column for each sales contact's first name and another for the last name. The following SQL statement would put them together:
SELECT CONCAT(name_first, ' ', name_last) AS Name FROM contacts;
This statement will display the first name, a space, and then the last name together in one column. The AS clause will change the column heading of the results to Name.
A less used concatenating function is CONCAT_WS(). It will put together columns with a separator between each. This can be useful when making data available for other programs. For instance, suppose we have a program that will import data, but it requires the fields to be separated by vertical bars. We could just export the data, or we could use a SELECT statement like the one that follows in conjunction with an interface written with an API language like Perl:
SELECT CONCAT_WS('|', col1, col2, col3) FROM table1;
The first element above is the separator. The remaining elements are the columns to be strung together.
If we want to format a long number with commas every three digits and a period for the decimal point (e.g., 100,000.00), we can use the function FORMAT() like so:
SELECT CONCAT('$', FORMAT(col5, 2)) FROM table3;
In this statement, the CONCAT() will place a dollar sign in front of the numbers found in the
col5 column, which will be formatted with commas by FORMAT(). The
2 within the FORMAT() stipulates two decimal places.
Occasionally, one will want to convert the text from a column to either all upper-case letters or all lower-case letters. In the example that follows, the output of the first column is converted to upper-case and the second to lower-case:
SELECT UCASE(col1), LCASE(col2) FROM table4;
When displaying data in forms, it's sometimes useful to pad the data displayed with zeros or dots or some other filler. This can be necessary when dealing with VARCHAR columns where the width varies to help the user to see the column limits. There are two functions that may be used for padding: LPAD() and RPAD().
SELECT RPAD(part_nbr, 8, '.') AS 'Part Nbr.', LPAD(description, 15, '_') AS Description FROM catalog;
In this SQL statement, dots are added to the right end of each part number. So a part number of "H200" will display as "H200....", but without the quotes. Each part's description will have under-scores preceding it. A part with a description of "brass hinge" will display as "brass hinge".
If a column is a CHAR data-type, a fixed width column, then it may be necessary to trim any leading or trailing spaces from displays. There are a few functions to accomplish this task. The LTRIM() function will eliminate any leading spaces to the left. So "
H200" becomes "
H200". For columns with trailing spaces, spaces on the right, RTRIM() will work: "
H500 " becomes "
H500". A more versatile trimming function, though, is TRIM(). With it one can trim left, right or both. Below are a few examples:
SELECT TRIM(LEADING '.' FROM col1), TRIM(TRAILING FROM col2), TRIM(BOTH '_' FROM col3), TRIM(col4) FROM table5;
In the first TRIM() clause, the padding component is specified; the leading dots are to be trimmed from the output of
col1. The trailing spaces will be trimmed off of
col2—space is the default. Both leading and trailing under-scores are trimmed from
col3 above. Unless specified, BOTH is the default. So leading and trailing spaces are trimmed from
col4 in the statement here.
When there is a need to extract specific elements from a column, MariaDB has a few functions that can help. Suppose a column in the table contacts contains the telephone numbers of sales contacts, including the area-codes, but without any dashes or parentheses. The area-code of each could be extracted for sorting with the LEFT() and the telephone number with the RIGHT() function.
SELECT LEFT(telephone, 3) AS area_code, RIGHT(telephone, 7) AS tel_nbr FROM contacts ORDER BY area_code;
In the LEFT() function above, the column telephone is given along with the number of characters to extract, starting from the first character on the left in the column. The RIGHT() function is similar, but it starts from the last character on the right, counting left to capture, in this statement, the last seven characters. In the SQL statement above, area_code is reused to order the results set. To reformat the telephone number, it will be necessary to use the SUBSTRING() function.
SELECT CONCAT('(', LEFT(telephone, 3), ') ', SUBSTRING(telephone, 4, 3), '-', MID(telephone, 7)) AS 'Telephone Number' FROM contacts ORDER BY LEFT(telephone, 3);
In this SQL statement, the CONCAT() function is employed to assemble some characters and extracted data to produce a common display for telephone numbers (e.g., (504) 555-1234). The first element of the CONCAT() is an opening parenthesis. Next, a LEFT() is used to get the first three characters of telephone, the area-code. After that a closing parenthesis, along with a space is added to the output. The next element uses the SUBSTRING() function to extract the telephone number's prefix, starting at the fourth position, for a total of three characters. Then a dash is inserted into the display. Finally, the function MID() extracts the remainder of the telephone number, starting at the seventh position. The functions MID() and SUBSTRING() are interchangeable and their syntax are the same. By default, for both functions, if the number of characters to capture isn't specified, then it's assumed that the remaining ones are to be extracted.
There are a few functions in MariaDB that can help in manipulating text. One such function is REPLACE(). With it every occurrence of a search parameter in a string can be replaced. For example, suppose we wanted to replace the title Mrs. with Ms. in a column containing the person's title, but only in the output. The following SQL would do the trick:
SELECT CONCAT(REPLACE(title, 'Mrs.', 'Ms.'), ' ', name_first, ' ', name_last) AS Name FROM contacts;
We're using the ever handy CONCAT() function to put together the contact's name with spaces. The REPLACE() function extracts each title and replaces Mrs. with Ms., where applicable. Otherwise, for all other titles, it displays them unchanged.
If we want to insert or replace certain text from a column (but not all of its contents), we could use the INSERT() function in conjunction with the LOCATE() function. For example, suppose another contacts table has the contact's title and full name in one column. To change the occurrences of Mrs. to Ms., we could not use REPLACE() since the title is embedded in this example. Instead, we would do the following:
SELECT INSERT(name, LOCATE(name, 'Mrs.'), 4, 'Ms.') FROM contacts;
The first element of the INSERT() function is the column. The second element which contains the LOCATE() is the position in the string that text is to be inserted. The third element is optional; it states the number of characters to overwrite. In this case, Mrs. which is four characters is overwritten with Ms. (the final element), which is only three. Incidentally, if 0 is specified, then nothing is overwritten, text is inserted only. As for the LOCATE() function, the first element is the column and the second the search text. It returns the position within the column where the text is found. If it's not found, then 0 is returned. A value of 0 for the position in the INSERT() function negates it and returns the value of name unchanged.
On the odd chance that there is a need to reverse the content of a column, there's the REVERSE() function. You would just place the column name within the function. Another minor function is the REPEAT() function. With it a string may be repeated in the display:
SELECT REPEAT(col1, 2) FROM table1;
The first component of the function above is the string or column to be repeated. The second component states the number of times it's to be repeated.
The function CHAR_LENGTH() is used to determine the number of characters in a string. This could be useful in a situation where a column contains different types of information of specific lengths. For instance, suppose a column in a table for a college contains identification numbers for students, faculty, and staff. If student identification numbers have eight characters while others have less, the following will count the number of student records:
SELECT COUNT(school_id) AS 'Number of Students' FROM table8 WHERE CHAR_LENGTH(school_id)=8;
The COUNT() function above counts the number of rows that meet the condition of the
SELECT ip_address FROM computers WHERE server='Y' ORDER BY ip_address LIMIT 3; +-------------+ | ip_address | +-------------+ | 10.0.1.1 | | 10.0.11.1 | | 10.0.2.1 | +-------------+
In the limited results above, the IP address 10.0.2.1 should be second. This happens because the column is being sorted lexically and not numerically. The function INET_ATON() will solve this sorting problem.
SELECT ip_address FROM computers WHERE server='Y' ORDER BY INET_ATON(ip_address) LIMIT 3;
Basically, the INET_ATON() function will convert IP addresses to regular numbers for numeric sorting. For instance, if we were to use the function in the list of columns in a SELECT statement, instead of the
WHERE clause, the address 10.0.1.1 would return 167772417, 10.0.11.1 will return 167774977, and 10.0.2.1 the number 167772673. As a complement to INET_ATON(), the function INET_ATON() will translate these numbers back to their original IP addresses.
MariaDB is fairly case insensitive, which usually is fine. However, to be able to check by case, the STRCMP() function can be used. It converts the column examined to a string and makes a comparison to the search parameter.
SELECT col1, col2 FROM table6 WHERE STRCMP(col3, 'text')=0;
If there is an exact match, the function STRCMP() returns 0. So if
col3 here contains "Text", it won't match. Incidentally, if
col3 alphabetically is before the string to which it's compared, a
-1 will be returned. If it's after it, a
1 is returned.
When you have list of items in one string, the SUBSTRING_INDEX() can be used to pull out a sub-string of data. As an example, suppose we have a column which has five elements, but we want to retrieve just the first two elements. This SQL statement will return them:
SELECT SUBSTRING_INDEX(col4, '|', 2) FROM table7;
The first component in the function above is the column or string to be picked apart. The second component is the delimiter. The third is the number of elements to return, counting from the left. If we want to grab the last two elements, we would use a negative two to instruct MariaDB to count from the right end.
There are more string functions available in MariaDB. A few of the functions mentioned here have aliases or close alternatives. There are also functions for converting between ASCII, binary, hexi-decimal, and octal strings. And there are also string functions related to text encryption and decryption that were not mentioned. However, this article has given you a good collection of common string functions that will assist you in building more powerful and accurate SQL statements.