jet/wiki/SELECT.md at 3e7277015d43491b5a803d5294c69ff08d6c2670

go-jet 3e7277015d Update SELECT wiki with set operators.

2019-07-02 10:31:52 +02:00

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SELECT statement is used to retrieve records from one or more tables in PostgreSQL. More about SELECT statement in postgres can be found at: https://www.postgresql.org/docs/11/sql-select.html

Following clauses are supported:

SELECT(expressions...) - expressions to form output rows of the SELECT statement.
DISTINCT() - remove all duplicate rows from result set
FROM(tableSource...) - specifies one or more source tables for the SELECT.
WHERE(condition) - only rows for which condition returns true will be selected.
GROUP BY(groupingElement, ...) - will condense into a single row all selected rows that share the same values for the grouped expressions.
HAVING(condition) - eliminates group rows that do not satisfy the condition
ORDER BY(orderBy, ...) - causes the result rows to be sorted according to the specified expression(s)
LIMIT(count) - specifies the maximum number of rows to return
OFFSET(start) - specifies the number of rows to skip before starting to return rows
FOR(lockMode) - how SELECT will lock rows as they are obtained from the table
Lock mode can be:
- jet.UPDATE()
- jet.NO_KEY_UPDATE()
- jet.SHARE()
- jet.KEY_SHARE()
Lock mode can be extended with following clauses:
- NOWAIT()
- SKIP_LOCKED()
UNION(select) / UNION_ALL(select) - computes the set union of the rows returned by the involved SELECT statements
INTERSECT(select) / INTERSECT_ALL(select) - computes the set intersection of the rows returned by the involved SELECT statements
EXCEPT(select) / EXCEPT_ALL(select) - computes the set of rows that are in the result of the left SELECT statement but not in the result of the right one

This list might be extended with feature Jet releases.

Examples per clause

1. SELECT clause

Sample SELECT clause written in Go:

   jet.SELECT(
        jet.Int(1).ADD(jet.Int(12)).SUB(jet.Int(21)),                               // arbitrary expression
        table.Film.Name,                                                            // column
        table.Customer.FirstName.CONCAT(table.Customer.LastName).AS("FullName")     // alias
   )

If jet and autogenerated table package, is dot "." imported, above statement will look more like native SQL:

   SELECT(
        Int(1).ADD(Int(12)).SUB(Int(21)),                               // arbitrary expression
        Film.Name,                                                      // column
        Customer.FirstName.CONCAT(Customer.LastName).AS("FullName")     // alias
   )

For the most of the wiki dot import is used.

Above SQL clause written in go will produce following raw SQL:

SELECT 1 + 12 - 21,
       film.name AS "film.name",                              --  
       customer.first_name || customer.last_name AS "FullName"

film.name AS "film.name" - column names are aliased by default. Alias is used during execution to map row result to appropriate model structure.

2. DISTINCT clause

SELECT(Film.Name).
.DISTINCT().

Raw SQL:

SELECT DISTINCT film.name

3. FROM clause

Go:
1).FROM(Film)
2).FROM(
      Film.
        INNER_JOIN(Language, Langauge.LanguageID.EQ(Film.FilmID))
  )
  
SQL:
1)FROM dvds.film
2)FROM Film 
       INNER JOIN Language ON (Language.LanguageID = Film.FilmID)

4. WHERE clause

Go:
.WHERE(Film.Length.GT(Int(150)))

SQL:
WHERE film.length > 150

5. GROUP BY clause

Go:
.GROUP_BY(Film.Length)

SQL:
GROUP BY film.length

6. HAVING clause

Go:
.HAVING(Film.Length.GT(Int(150))

SQL:
HAVING film.length > 150

7. ORDER BY clause

Go:
.ORDER_BY(Film.Length)

SQL:
ORDER BY film.length

8. LIMIT clause

Go:
.LIMIT(11)

SQL:
LIMIT 11

9. OFFSET clause

Go:
.OFFSET(11)

SQL:
OFFSET 11

9. FOR clause

Go:
.FOR(jet.NO_KEY_UPDATE().SKIP_LOCKED())

SQL:
FOR NO KEY UPDATE SKIP LOCKED

10. Set clauses (UNION, UNION_ALL, INTERSECT, INTERSECT_ALL, EXCEPT, EXCEPT_ALL)

Go:
SELECT(Payment.Amount).FROM(Payment)
UNION_ALL(SELECT(Payment.Amount).FROM(Payment))

Sql:
(
     (
          SELECT payment.amount AS "payment.amount"
          FROM dvds.payment
     )
     UNION
     (
          SELECT payment.amount AS "payment.amount"
          FROM dvds.payment
     )
);

Two forms of select statements in Jet

1. Classical select statement

Columns selected are before table source selected

SELECT(
    Payment.AllColumns,
    Customer.AllColumns,
).
FROM(Payment.
    INNER_JOIN(Customer, Payment.CustomerID.EQ(Customer.CustomerID))).
ORDER_BY(Payment.PaymentID.ASC()).
LIMIT(30)

2. Jet select statement

Table sources are before columns selected. There is no FROM.

Payment.
    INNER_JOIN(Customer, Payment.CustomerID.EQ(Customer.CustomerID))
SELECT(
    Payment.AllColumns,
    Customer.AllColumns,
).
ORDER_BY(Payment.PaymentID.ASC()).
LIMIT(30)

Second form is added, because sometimes feels more natural to first think about the tables of interest, and than about the columns.
Both forms produces exactly the same raw SQL.

Sub-queries

How to write SELECT statement with sub-queries?

Sub-queries are composed first:

// select film_id and title from film table that have 'R' rating.
rRatingFilms := Film.
    SELECT(
        Film.FilmID,
        Film.Title,
    ).
    WHERE(Film.Rating.EQ(enum.MpaaRating.R)).
    AsTable("rFilms")

AsTable("rFilms") - allows SELECT statements to be seen as table.

To use sub-query columns in SELECT statement expressions we have to export column from sub-query, using From method.

rFilmId := Film.FilmID.From(rRatingFilms)  //used for join condition

Now we can write:

query := Actor.
    INNER_JOIN(FilmActor, Actor.ActorID.EQ(FilmActor.FilmID)).
    INNER_JOIN(rRatingFilms, FilmActor.FilmID.EQ(rFilmId)).
    SELECT(
        Actor.AllColumns,
        FilmActor.AllColumns,
        rRatingFilms.AllColumns(),
    )

rRatingFilms.AllColumns(), - sub-query columns needed for projection can be exported with AllColumns() method. No need to export each one of them with From.

Debug SQL of above example:

SELECT actor.actor_id AS "actor.actor_id",
     actor.first_name AS "actor.first_name",
     actor.last_name AS "actor.last_name",
     actor.last_update AS "actor.last_update",
     film_actor.actor_id AS "film_actor.actor_id",
     film_actor.film_id AS "film_actor.film_id",
     film_actor.last_update AS "film_actor.last_update",
     "rFilms"."film.film_id" AS "film.film_id",
     "rFilms"."film.title" AS "film.title",
     "rFilms"."film.rating" AS "film.rating"
FROM dvds.actor
     INNER JOIN dvds.film_actor ON (actor.actor_id = film_actor.film_id)
     INNER JOIN (
          SELECT film.film_id AS "film.film_id",
               film.title AS "film.title",
               film.rating AS "film.rating"
          FROM dvds.film
          WHERE film.rating = 'R'
     ) AS "rFilms" ON (film_actor.film_id = "rFilms"."film.film_id");

What If sub-query projection is not a column?

For instance:

customersPayments := Payment.
    SELECT(
        Payment.CustomerID,
        SUMf(Payment.Amount).AS("amount_sum"),
    ).
    GROUP_BY(Payment.CustomerID).
    AsTable("customer_payment_sum")

customerId := Payment.CustomerID.From(customersPayments)

To export "amount_sum" from customersPayments sub-query we have to create column first with appropriate type and a name. Because SUMf produces float expression we will create FloatColumn with name of the column alias "amount_sum"

amountSum := FloatColumn("amount_sum").From(customersPayments)

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