Query and Execute API

Query

The query function sends a query to the server and returns the result. Non-data queries return a map with a tag:

(pg/query conn "create table test1 (id serial primary key, name text)")
{:command "CREATE TABLE"}

(pg/query conn "insert into test1 (name) values ('Ivan'), ('Huan')")
{:inserted 2}

Data queries return a vector of maps. This behaviour may be changed with reducers (see below).

(pg/query conn "select * from test1")
[{:name "Ivan", :id 1} {:name "Huan", :id 2}]

The SQL string might include several expressions concatenated with a semicolon. In this case, the result will be a vector of results:

(pg/query conn "insert into test1 (name) values ('Juan'); select * from test1")

[{:inserted 1}
 [{:name "Ivan", :id 1}
  {:name "Huan", :id 2}
  {:name "Juan", :id 3}]]

Use this feature wisely; don't try to do lots of things at once.

Important: the query function doesn't support parameters. You cannot run a query like these two below or similar:

(pg/query conn "select * from test1 where id = ?" 1)
;; or
(pg/query conn ["select * from test1 where id = ?" 1])

NEVER(!), NEVER(!!), NEVER(!!!) put parameters into a SQL string using str, format, or other functions that operate on strings. You will regret it one day. Use execute with parameters instead.

Execute

The execute function acts like query but has the following peculiarities:

• The SQL string cannot have many expressions concatenated with a semicolon. There must be a single expression (although the trailing semicolon is allowed).

• It may have parameters. The values for these parameters are passed separately. Unlike in JDBC, the parameters use dollar sign with a number, for example $1, $2, etc.

Here is how you can query a row by its primary key:

(pg/execute conn "select * from test1 where id = $1" {:params [2]})
;; [{:name "Huan", :id 2}]

The values are passed into the :params key; they must be a vector, or a list, or a lazy sequence. Passing a set is not recommended as it doesn't guarantee the order of the values.

This is how you insert values into a table using parameters:

(pg/execute conn
            "insert into test1 (name) values ($1), ($2), ($3)"
            {:params ["Huey" "Dewey" "Louie"]})
;; {:inserted 3}

Pay attention that the values are always a flat list. Imagine you'd like to insert rows with explicit ids:

(pg/execute conn
            "insert into test1 (id, name) values ($1, $2), ($3, $4), ($5, $6)"
            {:params [1001 "Harry" 1002 "Hermione" 1003 "Ron"]})
;; {:inserted 3}

The :params vector consists from flat values but not pairs like [1001 "Harry"]. For better readability, make a list of pairs and then flatten it:

(def pairs
  [[1001 "Harry"]
   [1002 "Hermione"]
   [1003 "Ron"]])

(flatten pairs)

;; (1001 "Harry" 1002 "Hermione" 1003 "Ron")

Since the parameters have explicit numbers, you can reference a certain value many times. The following query will create three agents Smith with different ids.

(pg/execute conn
            "insert into test1 (name) values ($1), ($1), ($1)"
            {:params ["Agent Smith"]})
;; {:inserted 3}

Both query and execute functions accept various options that affect data processing. Find their description in the next section.