I'm writing a query in MySQL wherein, i'm framing a case statement to get some values. However, i'm not getting any way to pass the column name dynamically in the 'then' statement of the case clause. Below is the code am trying.
SELECT CONCAT (COLUMN_NAME,(CASE WHEN
IS_NULL_FLAG = 'Y' AND COLUMN_TYPE = 'NUMBER' then '== ? (int ?) null:Convert.ToInt32(#COLUMN_NAME) AS #COLUMN_NAME'
else ''
end
) )FROM SCHEMA_REF WHERE SCHEMA_REF.ENTITY_NAME = 'CUSTOMERS1_REP' ORDER BY SCHEMA_ID;
Here while fetching the record, i'm trying to get the column name, but it is not getting passed.
Any idea on how can I pass the column name dynamically in the 'then' clause of the case statement.
Thanks,
Arjun
I understand the syntax here:
UPDATE table
SET column1 = expression1,
column2 = expression2,
...
WHERE conditions;
and here:
UPDATE table1
SET column1 = (SELECT expression1
FROM table2
WHERE conditions)
WHERE conditions;
...but what if I want to update
UPDATE table1
SET column1 = expression
if we get a particular result on a join between table1 and table2
which has WHERE conditions?
I can't seem to figure it out, and I get syntax errors in all of my attempts. Any advice greatly appreciated.
The syntax in MySQL is:
UPDATE table1 JOIN
table2
ON conditions
SET table1.column1 = table2.expression1
WHERE conditions;
What I do is write it as a SELECT statement first.
SELECT t.id
, t.col AS old_val
, s.expr AS new_val
FROM target_table t
JOIN source_table s
ON s.somecol = t.somecol
AND s.othercol < 1
WHERE s.something_else = 'abc'
In this example, t.col is the column that I (eventually) want to assign a new value to. (Used in the SELECT statement here, this just displays the value currently stored in the column.)
The expression s.expr represents the expression that returns the value I want to assign to col. This could be as simple as a column reference, or can be a more complex expression.
The rest of the statement are the normal FROM, JOIN, ON and WHERE clauses we're familiar with.
Once I have a SELECT statement that is working, I can convert that into an UPDATE statement by
replacing SELECT ... FROM (at the beginning of the statement) with the keyword UPDATE
adding a SET clause before the WHERE clause.
For example, I would convert the SELECT statement above to something like this, to assign the value of s.expr to t.col:
UPDATE target_table t
JOIN source_table s
ON s.somecol = t.somecol
AND s.othercol < 1
SET t.col = s.expr
WHERE s.something_else = 'abc'
This the approach that works for me. Writing it as a SELECT first allows me to test, muck with the conditions and expressions, and review to verify which rows are going to be updated, what values are being replaced, and what values are going to be assigned to each row.
I want to update two different columns from two different tables in mysql. This is the brief version of what I want to do.
In more detail I want to update a column from table 1 only if a condition is true. Otherwise I want to update a other column from table 1.
The same I want to do for table 2 and this should be all done in a single UPDATE statement.
I tried something like this:
UPDATE `grouped_messages`
JOIN `messages` ON (`messages`.group_msg_id = 1) SET
(CASE WHEN(`grouped_messages`.`conversation_partner1`=10) THEN `grouped_messages`.`deletion_status_partner1`=1
WHEN(`grouped_messages`.`conversation_partner2`=10) THEN `grouped_messages`.`deletion_status_partner2`=1 END),
(CASE WHEN(`messages`.`from`=10) THEN `messages`.`deletion_status_from`=1
WHEN(`messages`.`to`=10) THEN `messages`.`deletion_status_to`=1 END);
but this is not working. Does anybody know a solution, which would work for my case?
You can't do an assignment within a CASE expression. Your SET clause is invalid. It needs to be SET column = expression.
To have the column conditionally updated; that is, to have the column not updated, have the expression return the current value of the column.
Let's use an alias of g for grouped_messages.
When the condition you are checking is true, you want the SET clause to be equivalent to:
SET g.deletion_status_partner1 = 1
When the condition is not true, you want to assign the current value of the column to itself, so the SET clause to equivalent to:
SET g.deletion_status_partner1 = g.deletion_status_partner1
To get that, you could do something like this:
SET g.deletion_status_partner1
= CASE WHEN g.conversation_partner1 = 10
THEN 1
ELSE g.deletion_status_partner1 -- existing column value
END
To modify an additional column, you'd need to add to the SET clause
, g.deletion_status_partner2
= CASE WHEN some_other_condition
THEN new_value
ELSE g.deletion_status_partner2 -- existing column value
END
(I haven't checked anything else in your statement; I only highlight the most obvious issue, and how to get an update statement to do "conditional" assignment.)
i have a table named FullSchedule and it contains teachers names and days and times within each day, and i want to write a Query as the following question
select all fields from FullSchedule, if the teacher's name = 'SomeName' then add a column called Check with the value 'Found' else if not found put the value 'Not Found'
what is the correct Query for this question ? notice that the teacher's column contains different names but i want to check for one of them. simply making a schedule for each teacher.
I would have that column in my table with a predefined value, let's say it's 'Not found' or 'Not checked' and then I would set that variable with a query of the type:
UPDATE table_name
SET column1 = value1, column2 = value2...., columnN = valueN
WHERE [condition];
If that doesn't fit your needs, you can try a set query from a query or something like this
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();
}