Use Comparison Operators in SQL to Remove/Return Columns with Null Values
by Srinath MS
When you use the comparison operators in SQL with the Where clause, you can remove the columns with null values.
Use Comparison Operators in SQL to Remove/Return Columns with Null Values
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- What is DDL, DML and DCL? – If you look at the large variety of SQL commands, they can be divided into three large subgroups. Data Definition Language deals with database schemas and descriptions of how the data should reside in the database, therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL. DML deals with data manipulation, and therefore includes most common SQL statements such SELECT, INSERT, etc. Data Control Language includes commands such as GRANT, and mostly concerns with rights, permissions and other controls of the database system.
- How do you get the number of rows affected by query? – SELECT COUNT (user_id) FROM users would only return the number of user_id’s.
- If the value in the column is repeatable, how do you find out the unique values? – Use DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can also ask for a number of distinct values by saying SELECT COUNT (DISTINCT user_firstname) FROM users;
- How do you return the a hundred books starting from 25th? – SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.
- You wrote a search engine that should retrieve 10 results at a time, but at the same time you’d like to know how many rows there’re total. How do you display that to the user? – SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages LIMIT 1,10; SELECT FOUND_ROWS(); The second query (not that COUNT() is never used) will tell you how many results there’re total, so you can display a phrase “Found 13,450,600 results, displaying 1-10″. Note that FOUND_ROWS does not pay attention to the LIMITs you specified and always returns the total number of rows affected by query.
- How would you write a query to select all teams that won either 2, 4, 6 or 8 games? – SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)
- How would you select all the users, whose phone number is null? – SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
- What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps USING (user_id) – It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id
- How do you find out which auto increment was assigned on the last insert? – SELECT LAST_INSERT_ID() will return the last value assigned by the auto_increment function. Note that you don’t have to specify the table name.
- What does –i-am-a-dummy flag to do when starting MySQL? – Makes the MySQL engine refuse UPDATE and DELETE commands where the WHERE clause is not present.
- On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do? – What it means is that so of the data that you’re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.
- When would you use ORDER BY in DELETE statement? – When you’re not deleting by row ID. Such as in DELETE FROM techinterviews_com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table techinterviews_com_questions.
- How can you see all indexes defined for a table? – SHOW INDEX FROM techinterviews_questions;
- How would you change a column from VARCHAR(10) to VARCHAR(50)? – ALTER TABLE techinterviews_questions CHANGE techinterviews_content techinterviews_CONTENT VARCHAR(50).
- How would you delete a column? – ALTER TABLE techinterviews_answers DROP answer_user_id.
- How would you change a table to InnoDB? – ALTER TABLE techinterviews_questions ENGINE innodb;
- When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables? –
- VARCHARs with length less than 4 become CHARs
- CHARs with length more than 3 become VARCHARs.
- NOT NULL gets added to the columns declared as PRIMARY KEYs
- Default values such as NULL are specified for each column
- How do I find out all databases starting with ‘tech’ to which I have access to? – SHOW DATABASES LIKE ‘tech%’;
- How do you concatenate strings in MySQL? – CONCAT (string1, string2, string3)
- How do you get a portion of a string? – SELECT SUBSTR(title, 1, 10) from techinterviews_questions;
- What’s the difference between CHAR_LENGTH and LENGTH? – The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.
- How do you convert a string to UTF-8? – SELECT (techinterviews_question USING utf8);
- What do % and _ mean inside LIKE statement? – % corresponds to 0 or more characters, _ is exactly one character.
- What does + mean in REGEXP? – At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.
- How do you get the month from a timestamp? – SELECT MONTH(techinterviews_timestamp) from techinterviews_questions;
- How do you offload the time/date handling to MySQL? – SELECT DATE_FORMAT(techinterviews_timestamp, ‘%Y-%m-%d’) from techinterviews_questions; A similar TIME_FORMAT function deals with time.
- How do you add three minutes to a date? – ADDDATE(techinterviews_publication_date, INTERVAL 3 MINUTE)
- What’s the difference between Unix timestamps and MySQL timestamps? – Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.
- How do you convert between Unix timestamps and MySQL timestamps? – UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp, FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.
- What are ENUMs used for in MySQL? – You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’);
- How are ENUMs and SETs represented internally? - As unique integers representing the powers of two, due to storage optimizations.
Date: 22 April 2000
A SQL tutorial for people using Cold Fusion or ASP.
Covers basic functionality such as SELECT, DELETE,
INSERT, LIKE, UPDATE, WHERE
This Tutorial was made to give you some working knowledge of SQL.
It does not include advanced topics like stored procedures,etc.
If you want to use a database on you site then you MUST know SQL.
SQL stands for structured query language. It is pronounced sequel
(like a movie sequel.) SQL allows you to manipulate databases
through the internet. This tutorial will show you how to retrieve,
modify, delete and add new records to a database.
A basic sql statement is formed like this:
DIRECTIVE fields [WHERE ((something op something)
[conj] (somthing op something))];
text inside [ brackets ] is optional.
Directive - (see below)
fields - the fields to be used. If you want to use all, put an asterisk *
WHERE - see below
op - operator: = !=
conj - conjunction for having two or more criteria: AND,OR,NOT
A directive is a SQL command that tells the parser to do something.
The directives that will be discussed are SELECT, DELETE,
INSERT, UPDATE, WHERE, FROM, IS, LIKE.
Select records from the database
INSERT INTO VALUES
insert a new record
modifies a record
do . . . to records matching criteria
in what table in the database to perform the directive
criteria for WHERE that does something to all records where fieldname is LIKE %foobar% meananing do somethink to a record where the data in FIELDNAME being the name of the field is similar or contains foobar.
basically the same thing as =.
The WHERE directive performs SELECT,DELETE,INSERT,
UPDATE on all records that match the specifies criteria. it comes after
SELECT, DELETE, INSERT, UPDATE and before
SELECT * FROM table
To retrieve records we need to use the SELECT directive.
Then you need to specify the fields to retrieve inside brackets [ ]
or you can use an asterisk * to select all fields. The use the FROM
directive followed by the table name in which to get the records from.
As an example, I will take all fields from a table named examples.
SELECT * FROM examples;
To delete a record, you need to use the DELETE directive followed
my the fields to delete followed by FROM and the table name to delete
from plus WHERE and the criteria. As an example, I’ll delete all records
from a table called phonebook where the age of the person is under 18.
DELETE * FROM phonebook WHERE ((age < 18));
Adding New Records
To add a new record, you use the INSERT INTO directive followed
by the table name and then the fields to add data inside ( parenthesis )
and seperated by commas. Then you type VALUES followd by the data
that should go into the fields inside ( parenthesis ) and seperated by commas.
As an example i will place some foney data into a table called phonebook.
INSERT INTO phonebook (name,age,number)
VALUES ("John Doe","32","610-555-1212);
When modifying records, the UPDATE SET directive is used.
First type UPDATE, followed by the table name in which the records
to be updated exist. Then type SET followed by each fieldname & “=” &
the data to be put inside the field. Then you probably want to use WHERE
to only update certain records and not every one..
For multiple field modifications seperate each by a comma.
As an exampl, I will update phonebook with some new data.
UPDATE phonebook SET name=''Jane Doe'',age=''17'',phone=''484-555-1212'';
The LIKE directive is very usefull when doing searches.
Say i want to find the word foobar in a field named keywords in table whatever.
I would do it like this:
SELECT * FROM whatever WHERE keywords LIKE ''%foobar%'';
The percent % is used to represent any possible character,number.
So this would select all records that have foobar in their keywords field.
The IS directive is basically the same thing as equal to =.
You use it in place of LIKE if you only want to find the exact specified text.
SELECT * FROM whatever WHERE ID IS 10;
· You are not allowed to put quotes or apostrophes in SQL
when using it with Cold Fusion Server.
· In ASP you must use double apostrophes when wrapping text.
Quotes and single apostrophes are not allowed
— oOo —