pmin and pmax fail in mutate connected to SQL Server

@jnolis commented on Nov 28, 2017, 6:36 PM UTC:

Occasionally, I will need to use pmin or pmax to replace values above or below a threshold, for instance:

require(dplyr)
data_frame(X = 1:10) %>%
  mutate(Y = pmax(X,5))

However, when I use dbplyr to try and do this using a SQL Server table, I get the following error:

tbl(con,"OrderInfo") %>%
  mutate(Test = pmax(Revenue,0))

Error: <SQL> 'SELECT  TOP 10 "CustomerId", "OrderId", "ClientOrderId", "Channel", "OrderDate", "Revenue", "Cost", MAX("Revenue", 0.0) AS "Test"
FROM "OrderInfo"'
  nanodbc/nanodbc.cpp:1587: 42000: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]The MAX function requires 1 argument(s). 

As you can see, the SQL code generator assumes that I want to do a pairwise MAX, however SQL Server does not support that (only taking the maximum over a single column).

A cursory Stack Overflow search suggests that to do this operation a case statement could be used. I think either the SQL code generator should either be updated to do this, or an error should be generated at the time of calling the mutate.

This issue was moved by krlmlr from tidyverse/dplyr/issues/3227.

@edgararuiz commented on Dec 2, 2017, 3:31 AM UTC:

You're correct, pmax and pmin is not currently supported. So this is a feature request. Thank you for the SO link.

@DavisVaughan commented on Dec 23, 2017, 1:04 PM UTC:

@edgararuiz, You guys could take the approach I used for Access. It only works for two columns, but that is the most common case.

                   # pmin/pmax for 2 columns
                   pmin          = function(x, y) {
                     sql_expr(iif(UQ(x) <= UQ(y), !!x, !!y))
                   },

                   pmax          = function(x, y) {
                     sql_expr(iif(UQ(x) <= UQ(y), !!y, !!x))
                   },

@krlmlr commented on Jan 17, 2018, 9:14 AM UTC:

@DavisVaughan: Macro expansion is a slippery slope, what if x and y are complex expressions?

Out of curiosity: Which databases actually support pairwise MIN() and MAX()? Sqlite3 does, Postgres and MySQL don't. Maybe we should just remove the default translation of pmin and pmax and enable it only for the databases that support this?

@edgararuiz commented on Jan 17, 2018, 1:45 PM UTC:

You're right, we would enable it only for those who support it. I believe can also do pmin and pmax

@DavisVaughan commented on Jan 17, 2018, 2:46 PM UTC:

For completeness, Postgres and MySQL have GREATEST / LEAST which do what you want.
dbGetQuery(con, "SELECT GREATEST(drat, wt) FROM mtcars").
Works with any number of columns like pmax / pmin. This should probably be implemented right? (No such function for MSSQL)

Docs for Postgres at the bottom here

Docs for MySQL here

A stack overflow Q/A confirming the equivalence of Postgres GREATEST and SQLite pairwise max() here.

@hadley commented on Jun 7, 2018, 11:43 PM UTC:

Let's remove it from the default translation, and add to Postgres, MySQL, and SQLite.

Is it worth making the default translation an error?

@krlmlr commented on Jun 18, 2018, 8:46 AM UTC:

If we don't translate pmin() and pmax() (and apply dbplyr's default passthrough for unknown functions), the database gives a helpful error message. It didn't work before, so by changing the error message we don't introduce any unwanted behavior, I think.

[jyypma]

For reference, Oracle also uses LEAST and GREATEST.
https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions060.htm

[jyypma]

LEAST and GREATEST in PostgreSQL (and perhaps also MySQL and Oracle) may not provide the same implementation as pmin and pmax in R in the presence of NA values. See for instance the example below

dat <- data.frame(a = c(1, 2, 3, 4, 5, 6, 7, NA, NA), 
                  b = c(4, 5, 6, 1, 2, 3, NA, 8, NA))
dat %>% mutate(maxval = pmax(a, b), minval = pmin(a, b))

   a  b maxval minval
1  1  4      4      1
2  2  5      5      2
3  3  6      6      3
4  4  1      4      1
5  5  2      5      2
6  6  3      6      3
7  7 NA     NA     NA
8 NA  8     NA     NA
9 NA NA     NA     NA

The same result with LEAST and GREATEST from PostgreSQL gives

tbl_ref %>% mutate(maxval = sql("GREATEST(a,b)"), minval = sql("LEAST(a,b)"))

# Source:   lazy query [?? x 4]
# Database: postgres 10.0.3
      a     b maxval minval
  <dbl> <dbl>  <dbl>  <dbl>
1     1     4      4      1
2     2     5      5      2
3     3     6      6      3
4     4     1      4      1
5     5     2      5      2
6     6     3      6      3
7     7    NA      7      7
8    NA     8      8      8
9    NA    NA     NA     NA

[hadley]

We should probably make least() and greatest() the default translation. @jyypma In general, we can't provide 100% equivalent for NA vs NULL behaviour so the minor differences are not a problem.

[hadley]

• SQLite: https://stackoverflow.com/questions/2166690/sqlite-equivalent-of-postgresqls-greatest-function (uses max)
• MySQL: https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#function_greatest
• PostgreSQL: https://www.postgresql.org/docs/9.5/functions-conditional.html
• MS SQL: https://dba.stackexchange.com/questions/187090 — no equivalent, need to use CASE WHEN
• Oracle: https://www.techonthenet.com/oracle/functions/greatest.php