head.tbl_lazy: Subset the first rows

In dbplyr: A 'dplyr' Back End for Databases

Subset the first rows

Description

This is a method for the head() generic. It is usually translated to the LIMIT clause of the SQL query. Because LIMIT is not an official part of the SQL specification, some database use other clauses like TOP or ⁠FETCH ROWS⁠.

Note that databases don't really have a sense of row order, so what "first" means is subject to interpretation. Most databases will respect ordering performed with arrange(), but it's not guaranteed. tail() is not supported at all because the situation is even murkier for the "last" rows.

Usage

## S3 method for class 'tbl_lazy'
head(x, n = 6L, ...)

Arguments

x	A lazy data frame backed by a database query.
n	Number of rows to return
...	Not used.

Value

Another tbl_lazy. Use show_query() to see the generated query, and use collect() to execute the query and return data to R.

Examples

library(dplyr, warn.conflicts = FALSE)

db <- memdb_frame(x = 1:100)
db %>% head() %>% show_query()

# Pretend we have data in a SQL server database
db2 <- lazy_frame(x = 1:100, con = simulate_mssql())
db2 %>% head() %>% show_query()

dbplyr documentation built on May 29, 2024, 6:19 a.m.