I'm trying to modify a table inside my PostgreSQL database, but it says there is duplicate! what is the best way to find a duplicate value inside a table? kinda a select query?
Try Like This
SELECT count(column_name), column_name
from table_name
group by column_name having count(column_name) > 1;
If you try to change a value in a column that is part of the PRIMARY KEY or has a UNIQUE constraint and get this error there, then you should be able to find the conflicting row by
SELECT *
FROM your_table
WHERE conflicting_column = conflicting_value;
If conflicting_value is a character type, put it in single quotes (').
EDIT: To find out which columns are affected by the constraint, check this post.
First of all, determine which fields in your table have to be unique. This may be something marked as a Primary Key, a unique index based on one or more fields or a check constraint, again based on one or more fields.
Once you've done that, look at what you're trying to insert and work out whether it busts any of the unique rules.
And yes, SELECT statements will help you determine what's wrong here. Use those to determine whether you are able to commit the row.
Related
I have been trying to find an optimal solution to select unique values from each column. My problem is I don't know column names in advance since different table has different number of columns. So first, I have to find column names and I could use below query to do it:
select column_name from information_schema.columns
where table_name='m0301010000_ds' and column_name like 'c%'
Sample output for column names:
c1, c2a, c2b, c2c, c2d, c2e, c2f, c2g, c2h, c2i, c2j, c2k, ...
Then I would use returned column names to get unique/distinct value in each column and not just distinct row.
I know a simplest and lousy way is to write select distict column_name from table where column_name = 'something' for every single column (around 20-50 times) and its very time consuming too. Since I can't use more than one distinct per column_name, I am stuck with this old school solution.
I am sure there would be a faster and elegant way to achieve this, and I just couldn't figure how. I will really appreciate any help on this.
You can't just return rows, since distinct values don't go together any more.
You could return arrays, which can be had simpler than you may have expected:
SELECT array_agg(DISTINCT c1) AS c1_arr
,array_agg(DISTINCT c2a) AS c2a_arr
,array_agg(DISTINCT c2b) AS c2ba_arr
, ...
FROM m0301010000_ds;
This returns distinct values per column. One array (possibly big) for each column. All connections between values in columns (what used to be in the same row) are lost in the output.
Build SQL automatically
CREATE OR REPLACE FUNCTION f_build_sql_for_dist_vals(_tbl regclass)
RETURNS text AS
$func$
SELECT 'SELECT ' || string_agg(format('array_agg(DISTINCT %1$I) AS %1$I_arr'
, attname)
, E'\n ,' ORDER BY attnum)
|| E'\nFROM ' || _tbl
FROM pg_attribute
WHERE attrelid = _tbl -- valid, visible table name
AND attnum >= 1 -- exclude tableoid & friends
AND NOT attisdropped -- exclude dropped columns
$func$ LANGUAGE sql;
Call:
SELECT f_build_sql_for_dist_vals('public.m0301010000_ds');
Returns an SQL string as displayed above.
I use the system catalog pg_attribute instead of the information schema. And the object identifier type regclass for the table name. More explanation in this related answer:
PLpgSQL function to find columns with only NULL values in a given table
If you need this in "real time", you won't be able to archive it using a SQL that needs to do a full table scan to archive it.
I would advise you to create a separated table containing the distinct values for each column (initialized with SQL from #Erwin Brandstetter ;) and maintain it using a trigger on the original table.
Your new table will have one column per field. # of row will be equals to the max number of distinct values for one field.
For on insert: for each field to maintain check if that value is already there or not. If not, add it.
For on update: for each field to maintain that has old value != from new value, check if the new value is already there or not. If not, add it. Regarding the old value, check if any other row has that value, and if not, remove it from the list (set field to null).
For delete : for each field to maintain, check if any other row has that value, and if not, remove it from the list (set value to null).
This way the load mainly moved to the trigger, and the SQL on the value list table will super fast.
P.S.: Make sure to pass all you SQL from trigger to explain plan to make sure they use best index and execution plan as possible. For update/deletion, just check if old value exists (limit 1).
I have 2 tables. (table1, table2)
Now, I would like to do something along the lines of:
ALTER TABLE table1
ADD counter INT DEFAULT (Select COUNT(table2.car_type)
FROM table2
WHERE (table1.car_type = table2.car_type));
That's the closest syntax I can come up with, but just by looking at it I know it's wrong. Please tell me how to do it in ONE SQL statement (if possible).
The goal table (with counter implemented) is something like this: (separated columns by -)
table1:
pid-car_type-counter:
1-Honda-2
2-Toyota-3
3-Suzuki-1
4-Ferrari-0
5-Porsche-1
table2:
pid-car_type:
1-Honda
2-Toyota
3-Porsche
4-Honda
5-Suzuki
6-Toyota
7-Toyota
I don't think you can do this, at least through 11g. FTFM:
Restriction on Default Column Values: A DEFAULT expression cannot
contain references to PL/SQL functions or to other columns, the
pseudocolumns CURRVAL, NEXTVAL, LEVEL, PRIOR, and ROWNUM, or date
constants that are not fully specified.
I'm fairly certain you can't use a SELECT statement in a DEFAULT clause, either. There might be a way with virtual columns. Another alternative is to create a view.
I am struggling to retrieve the value of primary key for a table. We are using MS SQL Server 2005. The database was designed years back by somebody else (he didn't follow the normalization rules at all). He used Key (which is a keyword in sql server) as the column name for primary key of a table. So I cannot use query like this: select key from table_name where column2 = ?
Could anyone help to write a query to get the value of the primary key for a specific row something like this: select primary_key from tbale_name where column2 = ?
Yes you can, simply wrap column names in backticks:
select `key` from `table_name` where `column2` = ?
Alternatively, depending on your DB, you might use square brackets:
select table_name.[key] from table_name where table_name.[column2] = ?
Edit: I see you said "MS SQL". I think that one works with the square brackets. MySQL accepts the backtick syntax.
I have created a lookup table in Access to provide the possible values for a column. Now I need to update this column with the data it had before I converted the column. I am unable to figure out a SQL Query that will work. I keep getting the error "An UPDATE or DELETE query cannot contain a multi-valued field." My research has suggested that I just need to set the value of the column but this always updates 0 records:
UPDATE [table_name] SET [column_name].Value = 55 WHERE [table_name].ID = 16;
I know this query will work if I change it to update a text column, so it is definitely a problem with just this column.
If you're adding a value to your multi-valued field, use an append query.
INSERT INTO table_name( [column_name].Value )
VALUES (55)
WHERE ID = 16;
If you want to change one particular value which exists in your multi-valued field, use an UPDATE statement. For example, to change the 55 to 56 ...
UPDATE [table_name]
SET [column_name].Value = 56
WHERE [column_name].Value = 55 And ID = 16;
See Using multivalued fields in queries for more information.
I have figured this out! It certainly was counter-intuitive! You have to use an INSERT statement to do the update.
-- Update a record with a multi-valued field that has no value
INSERT INTO [table_name] ( [[column_name].[Value] )
VALUES(55)
WHERE [table_name].ID = 16;
This confused me because I was expecting an UPDATE statement. I think it actually inserts a record into a hidden table that is used to associate multiple values with this column.
I am working with Sharepoint, I created the tables as multi-value fields, ran into the error with my INSERT INTO statement, went back to Sharepoint to change to non-multi-value fields, but that didn't fix it.
Recreated the table without using multi-value fields, and the INSERT INTO worked just fine.
do not use the .value part
UPDATE [table_name] SET [column_name] = 55 WHERE [table_name].ID = 16;
INSERT INTO Quals (cTypes.[value])
SELECT Quals_ContractTypes.ContractType
FROM Quals_ContractTypes
WHERE (Quals.ID = Quals_ContractTypes.ID_Quals);
I gotta say I didn't understand very well your problem but I saw something strange in your query. Try this:
UPDATE [table_name] SET [column_name]= 55 WHERE [table_name].ID = 16;
UPDATE:
Look at this link: it has an example
UPDATE Issues
SET Issues.AssignedTo.Value = 10
WHERE (((Issues.AssignedTo.Value)=6)
AND ((Issues.ID)=8));
NOTES
You should always include a WHERE
clause that identifies only the
records that you want to update.
Otherwise, you will update records
that you did not intend to change. An
Update query that does not contain a
WHERE clause changes every row in the
table. You can specify one value to
change.
The Multi-Valued field refers to Access databases that have tables with columns, that allow you to select multiple values, like a Combo Checkbox list.
THOSE are the only Access types that SQL cannot work with. I've tested all Access lookup possibilities, including hard-coded values, and lookup tables. They work fine, but if you have a column that has the Allow Multiple select options, you're out of luck. Even using the INSERT INTO as mentioned below, will not work as you'll get a similar but different error, about INSERTing into multi-valued fields.
As mentioned it's best to avoid using such tables outside of Access, and refer to a table specifically for your external needs. Then write a macro/vba script to update the real tables with the data from the "auxiliary" table.
I have a table that looks a bit like this actors(forename, surname, stage_name);
I want to update stage_name to have a default value of
forename." ".surname
So that
insert into actors(forename, surname) values ('Stack', 'Overflow');
would produce the record
'Stack' 'Overflow' 'Stack Overflow'
Is this possible?
Thanks :)
MySQL does not support computed columns or expressions in the DEFAULT option of a column definition.
You can do this in a trigger (MySQL 5.0 or greater required):
CREATE TRIGGER format_stage_name
BEFORE INSERT ON actors
FOR EACH ROW
BEGIN
SET NEW.stage_name = CONCAT(NEW.forename, ' ', NEW.surname);
END
You may also want to create a similar trigger BEFORE UPDATE.
Watch out for NULL in forename and surname, because concat of a NULL with any other string produces a NULL. Use COALESCE() on each column or on the concatenated string as appropriate.
edit: The following example sets stage_name only if it's NULL. Otherwise you can specify the stage_name in your INSERT statement, and it'll be preserved.
CREATE TRIGGER format_stage_name
BEFORE INSERT ON actors
FOR EACH ROW
BEGIN
IF (NEW.stage_name IS NULL) THEN
SET NEW.stage_name = CONCAT(NEW.forename, ' ', NEW.surname);
END IF;
END
According to 10.1.4. Data Type Default Values no, you can't do that. You can only use a constant or CURRENT_TIMESTAMP.
OTOH if you're pretty up-to-date, you could probably use a trigger to accomplish the same thing.
My first thought is if you have the two values in other fields what is the compelling need for redundantly storing them in a third field? It flies in the face of normalization and efficiency.
If you simply want to store the concatenated value then you can simply create a view (or IMSNHO even better a stored procedure) that concatenates the values into a pseudo actor field and perform your reads from the view/sproc instead of the table directly.
If you absolutely must store the concatenated value you could handle this in two ways:
1) Use a stored procedure to do your inserts instead of straight SQL. This way you can receive the values and construct a value for the field you wish to populate then build the insert statement including a concatenated value for the actors field.
2) So I don't draw too many flames, treat this suggestion with kid gloves. Use only as a last resort. You could hack this behavior by adding a trigger to build the value if it is left null. Generally, triggers are not good. They add unseen cost and interactions to fairly simple interactions. You can, though, use the CREATE TRIGGER to update the actors field after a record is inserted or updated. Here is the reference page.
As of MySQL 8.0.13, you can use DEFAULT clause for a column which can be a literal constant or an expression.
If you want to use an expression then, simply enclose the required expression within parentheses.
(concat(forename," ",surname))
There are two ways to accomplish what you are trying to do as per my knowledge:
(important: consider backing up your table first before running below queries)
1- Drop the column "stage_name" all together and create a new one with DEFAULT constraint.
ALTER TABLE actors ADD COLUMN stage_name VARCHAR(20) DEFAULT (concat(forename," ",surname))
2- This will update newer entries in the column "stage_name" but not the old ones.
ALTER TABLE actors alter stage_name set DEFAULT (concat(forename," ",surname));
After that, if you need to update the previous values in the column "stage_name" then simply run:
UPDATE actors SET stage_name=(concat(forename," ",surname));
I believe this should solve your problem.