Add SQL-Builder.md wiki page.

wiki/SQL-Builder.md

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+
+## SQL Builder
+
+SQL Builder files are Go files, containing types necessary to write type safe SQL queries in Go. They are 
+autogenerated from database tables and enums. 
+
+### Table SQL Builder files
+
+Following rules are applied to generate table SQL Builder files:
+
+- for every table there is one Go SQL Builder file generated. File name is in snake case of the table name. 
+- every file contains one type - struct with nested jet.Table. Type name is a camel case of table name.
+- for every column of table there is a field column in SQL Builder table type. Field name is camel case of column name. 
+See below table for type mapping.
+- `AllColumns` is used as shorthand notation for list of all columns.
+- `MutableColumns` are all columns minus primary key columns _(Useful in INSERT or UPDATE statements)_.
+
+##### Mappings of database types to sql builder column types:
+
+| Database type(postgres)                         | Sql builder column type                                            |
+| ----------------------------------------------- | -------------------------------------------------- |
+| boolean                                         |  ColumnBool                                        |
+| smallint, integer, bigint                       |  ColumnInteger                                     |
+| real, numeric, decimal, double precision        |  ColumnFloat                                       |
+| date                                            |  ColumnDate                                        |
+| timestamp without time zone                     |  ColumnTimestamp                                   |
+| timestamp with time zone                        |  ColumnTimestampz                                  |
+| time without time zone                          |  ColumnTime                                        |
+| time with time zone                             |  ColumnTimez                                       |
+| enums, text, character, character varying       |                                                    |
+| bytea, uuid                                     |                                                    |
+| and all remaining types                         |  ColumnString                                      |
+
+### Example
+
+Sql table `address`:
+```
+CREATE TABLE dvds.address
+(
+    address_id serial NOT NULL DEFAULT,
+    address character varying(50) NOT NULL,
+    address2 character varying(50),
+    district character varying(20) NOT NULL,
+    city_id smallint NOT NULL,
+    postal_code character varying(10),
+    phone character varying(20) NOT NULL,
+    last_update timestamp without time zone NOT NULL DEFAULT now(),
+    CONSTRAINT address_pkey PRIMARY KEY (address_id)
+)
+```
+
+Part of the table sql builder file for table `address`. 
+
+```
+package table
+
+import (
+	"github.com/go-jet/jet"
+)
+
+var Address = newAddressTable()
+
+type AddressTable struct {
+	jet.Table
+
+	//Columns
+	AddressID  jet.ColumnInteger
+	Address    jet.ColumnString
+	Address2   jet.ColumnString
+	District   jet.ColumnString
+	CityID     jet.ColumnInteger
+	PostalCode jet.ColumnString
+	Phone      jet.ColumnString
+	LastUpdate jet.ColumnTimestamp
+
+	AllColumns     jet.ColumnList
+	MutableColumns jet.ColumnList
+}
+```
+
+### Enum SQL Builder files
+
+Following rules are applied to generate enum SQL Builder files:
+
+- for every enum there is one Go SQL Builder file generated. File name is in snake case of the enum name. 
+- every file contains one type. Type name is a camel case of enum name.
+- for every enum value there is a field in SQL Builder enum struct. Field name is camel case of enum value. Type is
+jet.StringExpression, meaning it can be used by string expressions methods.
+
+#### Example
+
+Enum `mpaa_rating`:
+```
+CREATE TYPE dvds.mpaa_rating AS ENUM
+    ('G', 'PG', 'PG-13', 'R', 'NC-17');
+```
+
+Enum SQL Builder file for `mpaa_rating`:
+```
+package enum
+
+import "github.com/go-jet/jet"
+
+var MpaaRating = &struct {
+	G    jet.StringExpression
+	PG   jet.StringExpression
+	PG13 jet.StringExpression
+	R    jet.StringExpression
+	NC17 jet.StringExpression
+}{
+	G:    jet.NewEnumValue("G"),
+	PG:   jet.NewEnumValue("PG"),
+	PG13: jet.NewEnumValue("PG-13"),
+	R:    jet.NewEnumValue("R"),
+	NC17: jet.NewEnumValue("NC-17"),
+}
+```