I'm a beginner MySQL user.
My teacher gave me a question to explain how this function works.
SELECT *
FROM TableName
WHERE ColumnName=ColumnName
Then, the result shows the rows that contain values in that column. (The null value is not appear)
I have no idea about it. I do searching for answer but most of it is talking about using IS NOT NULL.
You need to use ' = ' to select rows by checking an empty column right ?
If that's the case
You can simply use the below code
SELECT *
FROM TableName
WHERE ColumnName = ' '
The expression:
WHERE ColumnName = ColumnName
is comparing two values from the same column. This should be true in all cases, except when ColumnName contains a NULL value. So, you can equivalently write this as:
WHERE ColumnName IS NOT NULL
This version is more understandable and the preferred way to write the logic.
Related
Greetings Stackian Overflowers,
I'm trying to overwrite data in an existing column and do a STR_TO_DATE in the case it is already set.
Here's the code :
SELECT IF(columnA = '../../..' , '0000/00/00' , STR_TO_DATE(columnA, '%m/%d/%Y)
FROM tablename
WHERE extract_date = '2018-12-31';
In all reality, I must use this logic to make an update on the table. I tried this :
UPDATE tablename
IF columnA = '../../..' THEN
SET columnA = '0000-00-00' ELSE
SET columnA = STR_TO_DATE(columnA,'%m/%d/%Y);
Could use any help on the matter.
MySQL's REGEXP operator comes in handy here:
UPDATE tablename
SET date_col = CASE WHEN columnA REGEXP '[0-9]{2}/[0-9]{2}/[0-9]{4}'
THEN STR_TO_DATE(columnA, '%m/%d/%Y)
ELSE NULL END;
It doesn't make sense to update columnA from a string to a date, because the types don't match. A better strategy is to create a new column to store the date. Also, I recommend using NULL, not 0000-00-00, to represent a date string that could not be parsed.
Note that the regex I used is not foolproof and does not completely validate your date strings. But it should at least weed out string data which is really off.
I have a table in my database with the name contact. It has the following columns: name, mobile and twon. The problem is that I'm trying to get all the twon='Dubai', but when I execute my query it only retrieves 81000 rows but the total was 130000. The other remaining rows are not appearing in my query.
My Query:
SELECT * FROM `contact` WHERE `twon` = 'Dubai'
Can anyone tell me where I am going wrong or help me to access all the data from table?
= will fetch the record only if the column value is exactly 'dubai'.
Try with LIKE,TRIM and LOWER:
SELECT * FROM `contact` WHERE LOWER(TRIM(`twon`)) LIKE '%dubai%'
This query will fetch the records if twon column contains the word 'dubai'.
Different things could be wrong.
If you post examples of rows that should be returned, it would help us.
Case Sensitivity
If case sensitivity is a problem (e.g. 'dubai' or 'DUBAI' are not returned), you can use the LOWER function
SELECT * FROM `contact` WHERE LOWER(`twon`) = 'dubai'
Extra blanks
In some cases, extra blanks in the column content would fail, for instance ' Dubai' and ' Dubai '. You can use the TRIM function to get rid of trailing and leading blanks.
SELECT * FROM `contact` WHERE TRIM(`twon`) = 'Dubai'
Combination
Combining the two will work, too.
SELECT * FROM `contact` WHERE LOWER(TRIM(`twon`)) = 'dubai'
In a MySQL table i have a field, containing this value for a given record : "1908,2315,2316"
Here is my sql Query :
SELECT * FROM mytable WHERE 2316 IN (myfield)
I got 0 results!
I tried this :
SELECT * FROM mytable WHERE 2315 IN (myfield)
Still 0 results
And then i tried this :
SELECT * FROM mytable WHERE 1908 IN (myfield)
Surprisingly i obtained the record when searching with 1908! What should i do to also obtain the record when searching with 2315 and 2316 ? What am i missing ?
Thanks
You appear to be storing comma delimited values in a field. This is bad, bad, bad. You should be using a junction table, with one row per value.
But, sometimes you are stuck with data in a particular structure. If so, MySQL provides the find_in_set() functions.
SELECT *
FROM mytable
WHERE find_in_set(2316, myfield) > 0;
You can't use IN() over comma separated list of no.s its better to normalize your structure first for now you can use find_in_set to find results matching with comma separated string
SELECT * FROM mytable WHERE find_in_set('1908',myfield) > 0
This question has been asked and answered before, but I don't want to hunt for it; this question should be closed as a duplicate. But, to answer your question:
The commas in the string, the column value, are just characters. Those are part of the string. They aren't seen as "separators" between values in the SQL text. The way SQL sees it, the column contains a single value, not a "list" of values.
So, in your query, the IN (field) is equivalent to an equals comparison. It's equivalent to comparing to a string. For example:
... WHERE 2316 = '1908,2315,2316'
And those aren't equal, so the row isn't returned. The "surprisingly" finding of a match, in the case of:
... WHERE 1908 IN ('1908,2315,2316')
that's explained because that string is being evaluated in a numeric context. That is, the comparison returns true, because all of these also true:
... WHERE 1908 = '1908,2315,2316' + 0
... WHERE 1908 = '1908xyz' + 0
... WHERE 1908 = '1907qrs' + 1
(When evaluated in a numeric context, a string gets converted to numeric. It just happens that the string evaluates to a numeric value that equals the integer value it's being comparing to.)
You may be able to make use of the MySQL FIND_IN_SET function. For example:
... WHERE FIND_IN_SET(2316,'1908,2315,2316')
But, please seriously reconsider the design of storing comma separated list. I recommend Bill Karwin's "SQL Antipatterns" book...
http://www.amazon.com/SQL-Antipatterns-Programming-Pragmatic-Programmers/dp/1934356557
In mysql IN clause is utilized as
SELECT * FROM mytable WHERE column_name IN (set_of_values) ;
Mention column name instead of values
Please try
SELECT * FROM mytable WHERE LOCATE(CONCAT (',', 2316 ','), CONCAT (',',myfield,',' ) ) <>0
Just wondering, is there any quick way to count all the NULL values (from all columns) in a MySQL table?
Thanks for any idea!
If you want this done exclusively by MYSQL and without enumerating all of the columns take a look at this solution.
In this method you don't have to maintain the number of database columns by hard coding them. If your table schema will get modified this method will work, and won't require code change.
SET #db = 'testing'; -- database
SET #tb = 'fuzzysearch'; -- table
SET #x = ''; -- will hold the column names with ASCII method applied to retrieve the number of the first char
SET #numcolumns = 0; -- will hold the number of columns in the table
-- figure out how many columns we have
SELECT count(*) into #numcolumns FROM information_schema.columns where table_name=#tb and table_schema=#db;
-- we have to prepare some query from all columns of the table
SELECT group_concat(CONCAT('ASCII(',column_name,')') SEPARATOR ",") into #x from information_schema.columns where table_name=#tb and table_schema=#db;
-- after this query we have a variable separated with comma like
-- ASCII(col1),ASCII(col2),ASCII(col3)
-- we now generate a query to concat the columns using comma as separator (null values are omitted from concat)
-- then figgure out how many times the comma is in that substring (this is done by using length(value)-length(replace(value,',',''))
-- the number returned is how many non null columns we have in that column
-- then we deduct the number from the known number of columns, calculated previously
-- the +1 is added because there is no comma for single value
SET #s = CONCAT('SELECT #numcolumns - (length(CONCAT_WS(\',\',', #x, '))-length(replace(CONCAT_WS(\',\',', #x, '),\',\',\'\')) + 1) FROM ',#db,'.',#tb,';');
PREPARE stmt FROM #s;
EXECUTE stmt;
-- after this execution we have returned for each row the number of null columns
-- I will leave to you to add a sum() group call if you want to find the null values for the whole table
DEALLOCATE PREPARE stmt;
The ASCII is used to avoid reading, concatenating very long columns for nothing, also ASCII makes us safe for values where the first char is a comma(,).
Since you are working with reports, you may find this helpful as this can be reused for each table if you put in a method.
I tried to let as many comments as possible.
Let's split on pieces the above compact way (reverse way):
I wanted to end up having a query like this
SELECT totalcolumns - notnullcolumns from table; -- to return null columns for each row
While the first one is easy to calcule by running:
SELECT count(*) FROM information_schema.columns where table_name=#tb and table_schema=#db;
The second one the notnullcolumns is a bit of pain.
After a piece of examination of the functions available in MySQL, we detect that CONCAT_WS does not CONCAT null values
So running a query like this:
SELECT CONCAT_WS(",","First name",NULL,"Last Name");
returns: 'First name,Last Name'
This is good, we take rid of the null values from the enumeration.
But how do we get how many columns were actually concatenated?
Well that is tricky. We have to calculate the number of commas+1 to get the actually concatenated columns.
For this trick we used the following SQL notation
select length(value)-length(replace(value,',','')) +1 from table
Ok, so we have now the number of concatenated columns.
But the harder part is coming next.
We have to enumerate for CONCAT_WS() all values.
We need to have something like this:
SELECT CONCAT_WS(",",col1,col2,col3,col4,col5);
This is where we have to take use of the prepared statements, as we have to prepare an SQL query dynamically from yet unknown columns. We don't know how many columns will be in our table.
So for this we use data from information_schema columns table. We need to pass the table name, but also the database name, as we might have the same table name in separate databases.
We need a query that returns col1,col2,col3,col4,col5 to us on the CONCAT_WS "string"
So for this we run a query
SELECT group_concat(column_name SEPARATOR ",") into #x from information_schema.columns where table_name=#tb and table_schema=#db;
One more thing to mention. When we used the length() and replace() method to find out how many columns were concatenated, we have to make sure we do not have commas among the values. But also take note that we can have really long values in our database cells. For both of this trick we use method ASCII('value'), which will return the ASCII char of the first char, which cannot be comma and will return null for null columns.
That being said we can compact all this in the above comprehensive solution.
Something like
select id
, sum ( case when col1 is null then 1 else 0 end case ) col1
, sum ( case when col2 is null then 1 else 0 end case ) col2
, sum ( case when col3 is null then 1 else 0 end case ) col3
from contacts
group by id
Something like this (substitute COL_COUNT as appropriate):
select count(*) * COL_COUNT - count(col1) - count(col2) - ... - count(col_n) from table;
You should really do this using not only SQL, but the language which is at your disposal:
Obtain the metadata of each table - either using DESCRIBE table, or using a built-in metadata functionality in your db access technology
Create queries of the following type in a loop for each column. (in pseudo-code)
int nulls = 0;
for (String colmnName : columNames) {
query = "SELECT COUNT(*) FROM tableName WHERE " + columnName + " IS NULL";
Result result = executeQuery(query);
nulls += result.size();
}
I can't find the answer since searching mysql NOT in google is a nightmare (even with the quotes).
I need to make a query like this:
SELECT * FROM table WHERE field=value AND field2!=value2 AND field3!=value3
How it is done? Is it even possible?
SELECT * FROM table WHERE
((field = value) AND
(field2 <> value2) AND
(field3 <> value3))
If you're dealing with NULL, you have to do two things:
Use SET ANSI_NULLS ON
Declare NULL values to a dummy value.
SQL Cannot compare nulls.
To do that:
SET #value = ISNULL(#value, -1);
Yes, you can do exactly what you wrote, but use <> instead of !=
Perhaps the answer depends on what "value" is? For example, for an integer 123 value would be 123; for a string "foobar" value would be 'foobar'.
have you tried the <> operator
SELECT * FROM table WHERE field = value AND field2 <> value2
have you tried "<>"? it works in Delphi