Welcome! SQL is the Structured Query Language, a standard means of asking for data from databases, and is used to query sky survey data in astronomy research. This guide provides a brief overview of SQL and query examples are available with comments.
欢迎你学习使用巡天数据库中的SQL语言！SQL即结构化查询语言，它是一种从数据库中获取数据的标准化方法，可用于查询天文研究中的巡天数据。本手册包含了SQL的简要介绍，你还可以学习一些包含注释的具体查询示例。

Table of content

SQL Basics SQL基础知识

Database structure/ table descriptions 数据库与表的结构

Sky survey basics 巡天数据库常用字段

Query Examples

SQL Basics

The data in the database is contained in Tables, organized in columns and rows. In SQL you write queries to the database. A query is compound of the table columns you want to retrieve (the SELECT part), the table or tables that store the data (the FROM part) and the conditions to restrict the data you will obtain (the WHERE part).
数据库中的数据在表（table）中按照行(row)和列(column)组织起来。通过SQL你通过查询语句（query）从数据库中获取数据。查询语句包括要检索的表和列（SELECT子句）、存储数据的表（FROM子句）以及查询数据的限制条件（WHERE子句）。
E.g.

 SELECT <columns> FROM <tables> WHERE <conditions>

Mathematical and logical operators:

The AND operator displays a record if all conditions separated by AND are TRUE.
The OR operator displays a record if any of conditions separated by OR is TRUE.
The NOT operator displays a record if the condition(s) is NOT TRUE.

Database structure/ table descriptions

Below is a list of the most commonly used tables (and views) and a short description of them.

PhotoObj：stores information about the images of every object, including run, rerun, camcol, field, ra, dec, magnitudes and object flags. 存储每个目标的图像信息，包括run，rerun，camcol，field，ra，dec，magnitude和object flags等字段。

PlateX： stores information on the aluminum plates that the sky survey uses to take spectra, including their exposure times and reddening information. You will need to find the Plate and MJD in this table to look up an object's spectrum in the Get Spectra tool. 存储巡天中用于拍摄光谱的铝板信息，包括曝光时间和红化信息。你需要在此表中查询Plate和MJD从而在Get Spectra工具中查找目标天体的光谱。

SpecObj：stores information on objects' spectra, including redshifts and spectroscopic classifications. 存储有关目标天体光谱的信息，包括红移和光谱分类。

Neighbors： stores all PhotoObj pairs within 30 arcseconds. 在30角秒（arcsecond）内存储所有PhotoObj对。

Photoz：stores the photometrically estimated redshifts for all objects in the GalaxyTag view. 存储GalaxyTag视图中所有目标的光度估计红移。

Our sky survey data also contains several subsets of the PhotoObj table. PhotoPrimary contains only the "best" measurements of each object. Generally, it's better to use PhotoPrimary rather than PhotoObj, which contains both good and bad data. Star contains only data for stars, Galaxy contains only data for galaxies, and Unknown contains only data for objects classified as "unknown." These subsets are actually views rather than tables; you will get familiar with them later in through examples
我们的巡天数据还包含PhotoObj表的几个子集。 PhotoPrimary仅包含每个对象的“最佳”测量值。通常，最好使用PhotoPrimary而不是PhotoObj因为后者的数据质量不一。 Star仅包含星星的数据，Galaxy仅包含星系数据，而Unknown仅包含被归类为“未知”的对象的数据。这些子集实际上是视图而不是表。你将通过后面的示例进一步熟悉它们

Sky survey basics

ra: right ascension (sky longitude) 赤经

dec: declination (sky latitude) 赤纬

g magnitude: model magnitude in g filter 在g波段的星等

Query Examples

Example 1
-- Find unique objects in an RA/Dec box. 查询一个赤经/赤纬坐标区域内具有唯一性的目标。
--Table used: PhotoObj (PhotoPrimary is a view created based on PhotoObj)

SELECT TOP 100 objID,     -- the SELECT TOP clause is used to specify the number of records to return 
    field, ra, dec              
FROM PhotoPrimary       -- selecting from a view which contains only unique objects can avoid duplicates in the result 
WHERE
    ra > 185. and ra < 185.1 and dec > 15. and dec < 15.1

Example 2
-- Find all galaxies in a certain area of the sky that meet specific criteria in their measured parameters. 查询天区中参数符合特定判据的所有星系。
-- Table used: Galaxy

SELECT TOP 100 objID, ra, dec, cModelMag_g  --the cModelMag_g column contains “model  magnitude in g filter” 
FROM Galaxy 
WHERE ra BETWEEN 178 AND 180     --The BETWEEN operator selects values within a given range. 
  AND dec < 0
  AND cModelMag_g BETWEEN 18 AND 19
 

Example 3
-- This query looks for spectra of quasars and shows the date and time at which each spectrum was taken. 查询类星体的光谱，并显示每个光谱的拍摄日期和时间。
-- Table used: SpecPhoto (a view which is a pre-computed join of the most commonly-searched fields in both the SpecObj view that contains spectroscopy data and the PhotoObj view that contains photometry data)

SELECT TOP 100
sp.objID, sp.ra, sp.dec,
px.taiBegin, px.taiEnd,  
FROM specPhoto AS sp         -- aliases (names) are used to give a table, or a column a temporary name [principle]
JOIN plateX AS px             -- a JOIN clause is used to combine rows from two or more tables, based on key variable they have in common [principle]
ON sp.plateID = px.plateID
WHERE
(sp.class='QSO')
         

Example 4
-- This query counts the number of spectra of each spectral classification (galaxy, quasar, star). 查询每种光谱分类（星系，类星体，恒星）的光谱数。
-- Table used: SpecObj

SELECT class, count(*)    -- the count(*) statement returns the number of all records that meet specific search criteria
FROM SpecObj 
GROUP BY class            -- The GROUP BY statement groups results into groups (categories) based on the value of a data column.
  

Example 5
-- This query finds all objects within 30 arcseconds of one another that have very similar colors.查询彼此角距离小于30角秒且颜色相近的所有目标。
-- Table used: PhotoPrimary, Neighbors

SELECT TOP 10 P.ObjID                                  
FROM    PhotoPrimary AS P                              
JOIN Neighbors AS N ON P.ObjID = N.ObjID          
JOIN PhotoPrimary AS L ON L.ObjID = N.NeighborObjID     
WHERE   
P.ObjID < L. ObjID                                    
and abs((P.u-P.g)-(L.u-L.g))<0.05    -- if two objects’ color ratios u-g, g-r, r-I are less than 0.05m, they have a similar spectra 
and abs((P.g-P.r)-(L.g-L.r))<0.05     
and abs((P.r-P.i)-(L.r-L.i))<0.05
and abs((P.i-P.z)-(L.i-L.z))<0.05    -- the abs() function returns the absolute value of a number. 
 

Example 6
-- This query lists the number of each type of object observed by each special spectroscopic observation program. 查询每个光谱观测项目观测到的每种目标的数量。
-- Table used: SpecObjAll, PlateX

SELECT plate.programname, class,
COUNT(specObjId) AS numObjs    -- The COUNT() function returns the number of rows that matches a specified criteria.
FROM SpecObjAll
JOIN PlateX AS plate ON plate.plate = specObjAll.plate 
GROUP BY plate.programname, class     -- The GROUP BY statement sorts results into groups (categories) based on the value of a data column. 
ORDER BY plate.programname, class     -- the ORDER BY keyword is used to sort the result-set in ascending or descending order