seems like a stupid question...
I have a mysql table where I want to modify column A to a number 0 or 1 depending on the condition of another column B
So: if( B > 500 ) A = 1 ELSE A = 0
Column A = INT
Column B = DOUBLE
How do you do something like this in sql?
Thanks,
Erik
Try the following statement,
UPDATE tableName
SET A = (B > 500)
SQLFiddle Demo
(B > 500) is a boolean arithmetic in mysql which returns 1 and 0 for true and false , respectively.
You can also use CASE for much more RDBMS friendly,
UPDATE tableName
SET A = CASE WHEN B > 500 THEN 1 ELSE 0 END
Related
I would like to do the following.
Update a field based on the value of another field like
update table set if(fielda=1){fieldb=2 fieldc=3}else{fieldd=2 fielde=3}
I know this is not valid mysql but its the best way for me to describe the problem.
update table set
b = case when a = 1 then 2 else b end,
c = case when a = 1 then 3 else c end,
d = case when a = 1 then d else 2 end,
e = case when a = 1 then e else 3 end
edit
according to your comment try this:
update table set
datefield_a = case when field_a = 1 then now() else datefield_a end,
datefield_b = case when field_a <> 1 then now() else datefield_b end
I think this syntax will achieve the result you attempted to specify.
UPDATE mytable
SET fieldb = CASE WHEN fielda = 1 THEN 2 ELSE fieldb END
, fieldc = CASE WHEN fielda = 1 THEN 3 ELSE fieldc END
, fieldd = CASE WHEN fielda = 1 THEN fieldd ELSE 2 END
, fielde = CASE WHEN fielda = 1 THEN fielde ELSE 3 END
The "trick" here is that we are updating all four columns, but in some "cases", we are assigning the current value of the column back to the column, resulting in no real change to the column value. (Once you get your mind bent around that idea, it's pretty easy.)
With MySQL, we do have a handy IF function (not available in most other RDBMS) that we can use to abbreviate that a bit, and achieve the same thing:
UPDATE mytable
SET fieldb = IF(fielda = 1, 2, fieldb)
, fieldc = IF(fielda = 1, 3, fieldc)
, fieldd = IF(fielda = 1, fieldd, 2)
, fielde = IF(fielda = 1, fielde, 3)
The pain is that you still have to repeat that same conditional test multiple times.
A single scan through the table (like these statements do), and getting all those assignments done in one fell swoop is going to be faster (and more efficient) than breaking this up and doing the assignments piecemeal using multiple statements.
I need to make a select like,
Select * from table WHERE column = x if column != -1
but i have no idea for now.
Anyone know or made in past something like that?
Thanks.
You should also write like this,
Select * from table
WHERE
1 = case when column != -1 then
case when column = x then 1 else 0 end
else 1 end
You can utilize case when in where clause.
Similarly you can add more conditional criteria like,
Select * from table
WHERE
1 = case when column != -1 then
case when column = x then 1 else 0 end
else 1 end
AND
1 = case when column1 [conditional operator] value then
case when column1 = xx then 1 else 0 end
else 1 end
This is just an example how you can integrate more conditional criteria together, even though you can add more case when in else part even.
Update myTable SET field = 1 (if field = 0 or if field is null) where myid = 12345;
Update myTable SET field = 0 (if field = 1) where myid = 12345;
What is the best way to transform this Pseudocode in proper SQL for Oracle and MySQL?
You could simply use the modulo like this:
UPDATE myTable SET field = (field + 1) % 2 WHERE myId = 12345;
Due to the lack of a real boolean in both DBMS you need a case statement:
update myTable
set the_column = case when the_column = 1 then 0 else 1 end
where myId = 12345;
This assumes that the column never has different values than 0 and 1
I would like to do the following.
Update a field based on the value of another field like
update table set if(fielda=1){fieldb=2 fieldc=3}else{fieldd=2 fielde=3}
I know this is not valid mysql but its the best way for me to describe the problem.
update table set
b = case when a = 1 then 2 else b end,
c = case when a = 1 then 3 else c end,
d = case when a = 1 then d else 2 end,
e = case when a = 1 then e else 3 end
edit
according to your comment try this:
update table set
datefield_a = case when field_a = 1 then now() else datefield_a end,
datefield_b = case when field_a <> 1 then now() else datefield_b end
I think this syntax will achieve the result you attempted to specify.
UPDATE mytable
SET fieldb = CASE WHEN fielda = 1 THEN 2 ELSE fieldb END
, fieldc = CASE WHEN fielda = 1 THEN 3 ELSE fieldc END
, fieldd = CASE WHEN fielda = 1 THEN fieldd ELSE 2 END
, fielde = CASE WHEN fielda = 1 THEN fielde ELSE 3 END
The "trick" here is that we are updating all four columns, but in some "cases", we are assigning the current value of the column back to the column, resulting in no real change to the column value. (Once you get your mind bent around that idea, it's pretty easy.)
With MySQL, we do have a handy IF function (not available in most other RDBMS) that we can use to abbreviate that a bit, and achieve the same thing:
UPDATE mytable
SET fieldb = IF(fielda = 1, 2, fieldb)
, fieldc = IF(fielda = 1, 3, fieldc)
, fieldd = IF(fielda = 1, fieldd, 2)
, fielde = IF(fielda = 1, fielde, 3)
The pain is that you still have to repeat that same conditional test multiple times.
A single scan through the table (like these statements do), and getting all those assignments done in one fell swoop is going to be faster (and more efficient) than breaking this up and doing the assignments piecemeal using multiple statements.
I want to update a table in MySQL like this:
UPDATE Table
SET A = '20' IF A > 20
SET A = A IF A < 20
SET A = 0 IF A <= 1
WHERE A IS NOT NULL;
But the above SQL is not valid Syntax. I also tried this:
UPDATE table
SET A = IF(A > 20, 20, IF(A < 20, A, 0));
But is also invalid Syntax. How do I use an if statement in an update query like this?
I think you were 99% there:
UPDATE table
SET A = IF(A > 20, 20, IF(A < 20 && A > 1, A, 0))
WHERE A IS NOT NULL;
Add the && A > 1 to the second IF statement and your third condition is satisfied.
Edit:
Per #Andre's comment to the question and the suggestion that the nested IF is difficult to read, you could also do this as a couple of queries that don't do any unnecessary work and are readable:
UPDATE table SET A = 20 WHERE A > 20;
UPDATE table SET A = 0 WHERE A <= 1;
When A is NULL, it will not meet either of these conditions, and thus eliminates the need to specify that A not be NULL.
Next, there's no need for the third condition as #Andre suggested. If A is between 1 and 20, it gets left as-is.
Finally, setting A to 0 where A is less than or equal to 1 seems unusual. Values of 1 will be changed to 0. If you intend to simply set values less than 1 (including negative values) to 0, then you should swap < for <=.
UPDATE Table
SET A = Case
When A > 20 Then 20
When A <= 1 Then 0
End
WHERE A IS NOT NULL and ( A > 20 or A <= 1 )
or more simply, 2 statements
UPDATE Table
SET A = 20
where A > 20;
UPDATE Table
SET A = 0
where A <= 1;
With the help of below mentioned query you can update salary field based on conditions. It's a single line update query to update multiple rows in a table based on conditions.
UPDATE table
SET salary = Case
WHEN designation='developer' THEN salary + salary*0.20
WHEN designation='manager' THEN salary + salary*0.30
END