SQL VISUAL QUICKSTART GUIDE- P47 ppt

SELECT datediff"d",date1,date2 AS days FROM SELECT pubdate as date1 FROM titles WHERE title_id = 'T05' t1, SELECT pubdate as date2 FROM titles WHERE title_id = 'T06' t2; -- Count the mon

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Microsoft Access

Access metadata are available graphically

through the Design View of each database

object and programmatically through the

Visual Basic for Applications (VBA) or C#

language Access also creates and maintains

hidden system tables in each database

To show system tables:

◆ In Access 2003 or earlier, choose Tools >

Options > View tab > check System

Objects.

or

In Access 2007 or later, choose Microsoft

Office button > Access Options >

Current Database (in the left pane) >

scroll to Navigation > click Navigation

Options > check Show System Objects.

The system tables begin with MSys and are

commingled with the database’s other

tables You can open and query them as you

would ordinary tables The most interesting

system table is MSysObjects , which catalogs

all the objects in the database Listing 15.39

lists all the tables in the current database.

Note that system tables don’t have to be

visi-ble to be used in queries.

Microsoft SQL Server

SQL Server metadata are available through

the schema INFORMATION_SCHEMA and via

sys-tem stored procedures (Listing 15.40).

440

Listing 15.39 List the tables in the current Access

database To list queries instead, change Type = 1to

Type = 5.

SELECT Name FROM MSysObjects WHERE Type = 1;

Listing

Listing 15.40 Metadata statements and commands

for Microsoft SQL Server.

List the databases

exec sp_helpdb;

List the schemas

SELECT schema_name FROM information_schema.schemata;

List the tables (Method 1)

SELECT * FROM information_schema.tables WHERE table_type = 'BASE TABLE'

AND table_schema = 'schema_name';

exec sp_tables;

Describe a table (Method 1)

SELECT * FROM information_schema.columns

WHERE table_catalog = 'db_name' AND table_schema = 'schema_name' AND table_name = 'table_name';

exec sp_help table_name;

Listing

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Oracle metadata are available through data dictionary views and via sqlplus

(Listing 15.41) To list data dictionary

views, run this query in sqlplus : SELECT table_name, comments FROM dictionary

ORDER BY table_name;

For a list of Oracle databases (instances) in Unix or Linux, look in the file oratab located

in the directory /etc or /var/opt/oracle

In Windows, run this command at a com-mand prompt:

net start | find /i "OracleService"

Or choose Start > Run (Windows logo key+R), type services.msc , press Enter, and then inspect the Services list for entries that

begin with OracleService.

441

for Oracle.

List the schemas (users)

SELECT *

FROM all_users;

List the tables

SELECT table_name

FROM all_tables

WHERE owner = 'user_name';

SELECT *

FROM all_tab_columns

WHERE owner = 'user_name'

AND table_name = 'table_name';

Describe a table (Method 2, in sqlplus)

DESCRIBE table_name;

Listing

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IBM DB2

DB2 metadata are available through the

system catalog SYSCAT and via db2

(Listing 15.42).

442

for IBM DB2.

List the databases (in db2)

LIST DATABASE DIRECTORY;

List the schemas

SELECT schemaname FROM syscat.schemata;

SELECT tabname FROM syscat.tables

WHERE tabschema = 'schema_name';

List the tables (Method 2, in db2) LIST TABLES;

List the tables (Method 3, in db2)

LIST TABLES FOR SCHEMA schema_name;

SELECT * FROM syscat.columns

WHERE tabname = 'table_name' AND tabschema = 'schema_name';

Describe a table (Method 2, in db2)

DESCRIBE TABLE table_name SHOW DETAIL;

Listing

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MySQL metadata are available through the schema INFORMATION_SCHEMA and via mysql

(Listing 15.43).

443

Listing 15.43 Metadata statements and commands for

MySQL.

List the databases (Method 1)

SELECT schema_name

FROM information_schema.schemata;

List the databases (Method 2, in mysql)

SHOW DATABASES;

SELECT table_name

FROM information_schema.tables

WHERE table_schema = 'db_name';

List the tables (Method 2, in mysql)

SHOW TABLES;

SELECT *

FROM information_schema.columns

WHERE table_schema = 'db_name'

AND table_name = 'table_name';

Describe a table (Method 2, in mysql)

DESCRIBE table_name;

Listing

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PostgreSQL metadata are available through

the schema INFORMATION_SCHEMA and via

psql (Listing 15.44).

444

for PostgreSQL.

List the databases (Method 1)

psql list

List the databases (Method 2, in psql)

\l

List the schemas

SELECT schema_name FROM information_schema.schemata;

SELECT table_name FROM information_schema.tables

WHERE table_schema = 'schema_name';

List the tables (Method 2, in psql)

\dt

SELECT * FROM information_schema.columns

WHERE table_schema = 'schema_name' AND table_name = 'table_name';

Describe a table (Method 2, in psql)

\d table_name;

Listing

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Working with Dates

As pointed out in “Getting the Current Date and Time” and “Performing Datetime and Interval Arithmetic” in Chapter 5, DBMSs provide their own extended (nonstandard) functions for manipulating dates and times This section explains how to use these

built-in functions to do simple date arithmetic.

The queries in each listing:

◆ Extract parts (hour, day, month, and so on) of the current (system) datetime and return them as numbers.

◆ Add and subtract intervals of days, months, and years from a date.

◆ Count the days between two dates in dif-ferent rows of the same column The result is positive, zero, or negative depending on whether the first date falls before, on, or after the second date.

◆ Count the months between the earliest and latest dates in the same column.

Microsoft Access

The function datepart() extracts the speci-fied part of a datetime now() returns the current (system) date and time dateadd() adds a specified time interval to a date.

datediff() returns the number of speci-fied time intervals between two dates

(Listing 15.45).

■ Alternatives to datepart() are the extraction functions second() , day() , month() , and so on.

445

Listing 15.45 Working with dates in Microsoft Access.

Extract parts of the current datetime

SELECT

datepart("s", now()) AS sec_pt,

datepart("n", now()) AS min_pt,

datepart("h", now()) AS hr_pt,

datepart("d", now()) AS day_pt,

datepart("m", now()) AS mon_pt,

datepart("yyyy",now()) AS yr_pt;

Add or subtract days, months, and years

SELECT

dateadd("d", 2,pubdate) AS p2d,

dateadd("d", -2,pubdate) AS m2d,

dateadd("m", 2,pubdate) AS p2m,

dateadd("m", -2,pubdate) AS m2m,

dateadd("yyyy", 2,pubdate) AS p2y,

dateadd("yyyy",-2,pubdate) AS m2y

FROM titles

WHERE title_id = 'T05';

Count the days between two dates

SELECT datediff("d",date1,date2) AS days

FROM

(SELECT pubdate as date1

FROM titles

WHERE title_id = 'T05') t1,

FROM titles

WHERE title_id = 'T06') t2;

Count the months between two dates

SELECT datediff("m",date1,date2) AS months

FROM

(SELECT

MIN(pubdate) AS date1,

MAX(pubdate) AS date2

FROM titles) t1;

Listing

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Microsoft SQL Server

The function datepart() extracts the

speci-fied part of a datetime getdate() returns

the current (system) date and time.

dateadd() adds a specified time interval to a

date datediff() returns the number of

specified time intervals between two dates

(Listing 15.46).

■ Alternatives to datepart() are the

extraction functions day() , month() ,

and year()

446

Listing 15.46 Working with dates in Microsoft SQL

Server.

Extract parts of the current datetime SELECT

datepart("s", getdate()) AS sec_pt, datepart("n", getdate()) AS min_pt, datepart("hh", getdate()) AS hr_pt, datepart("d", getdate()) AS day_pt, datepart("m", getdate()) AS mon_pt, datepart("yyyy",getdate()) AS yr_pt;

Add or subtract days, months, and years SELECT

dateadd("d", 2,pubdate) AS p2d, dateadd("d", -2,pubdate) AS m2d, dateadd("m", 2,pubdate) AS p2m, dateadd("m", -2,pubdate) AS m2m, dateadd("yyyy", 2,pubdate) AS p2y, dateadd("yyyy",-2,pubdate) AS m2y FROM titles

Count the days between two dates SELECT datediff("d",date1,date2) AS days FROM

(SELECT pubdate as date1 FROM titles

WHERE title_id = 'T05') t1, (SELECT pubdate as date2 FROM titles

Count the months between two dates SELECT datediff("m",date1,date2) AS months FROM

(SELECT MIN(pubdate) AS date1, MAX(pubdate) AS date2 FROM titles) t1;

Listing

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The function to_char() converts a datetime

to a character value in the given format.

to_number() converts its argument to a number sysdate returns the current (sys-tem) date and time The standard addition and subtraction operators add and subtract days from a date add_months() adds a speci-fied number of months to a date.

Subtracting one date from another yields the number of days between them.

months_between() returns the number of

months between two dates (Listing 15.47).

447

Listing 15.47 Working with dates in Oracle.

SELECT

to_number(to_char(sysdate,'ss'))

AS sec_pt,

to_number(to_char(sysdate,'mi'))

AS min_pt,

to_number(to_char(sysdate,'hh24'))

AS hr_pt,

to_number(to_char(sysdate,'dd'))

AS day_pt,

to_number(to_char(sysdate,'mm'))

AS mon_pt,

to_number(to_char(sysdate,'yyyy'))

AS yr_pt

FROM dual;

SELECT

pubdate+2 AS p2d,

pubdate-2 AS m2d,

add_months(pubdate,+2) AS p2m,

add_months(pubdate,-2) AS m2m,

add_months(pubdate,+24) AS p2y,

add_months(pubdate,-24) AS m2y

FROM titles

SELECT date2 - date1 AS days

FROM

FROM titles

SELECT months_between(date2,date1) AS months

FROM

(SELECT

FROM titles) t1;

Listing

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IBM DB2

The functions second() , day() , month() ,

and so on, extract part of a datetime

current_timestamp returns the current

(system) date and time The standard

addition and subtraction operators add

and subtract time intervals from a date.

days() converts a date to an integer serial

number (Listing 15.48).

448

Listing 15.48 Working with dates in IBM DB2.

Extract parts of the current datetime SELECT

second(current_timestamp) AS sec_pt, minute(current_timestamp) AS min_pt, hour(current_timestamp) AS hr_pt, day(current_timestamp) AS day_pt, month(current_timestamp) AS mon_pt, year(current_timestamp) AS yr_pt FROM SYSIBM.SYSDUMMY1;

Add or subtract days, months, and years SELECT

pubdate + 2 DAY AS p2d, pubdate - 2 DAY AS m2d, pubdate + 2 MONTH AS p2m, pubdate - 2 MONTH AS m2m, pubdate + 2 YEAR AS p2y, pubdate - 2 YEAR AS m2y FROM titles

Count the days between two dates SELECT days(date2) - days(date1) AS days FROM

(SELECT pubdate as date1 FROM titles

WHERE title_id = 'T05') t1, (SELECT pubdate as date2 FROM titles

Count the months between two dates SELECT

(year(date2)*12 + month(date2)) -(year(date1)*12 + month(date1))

AS months FROM

(SELECT MIN(pubdate) AS date1, MAX(pubdate) AS date2 FROM titles) t1;

Listing

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The function date_format() formats a datetime according to the specified format.

current_timestamp returns the current (system) date and time The standard addi-tion and subtracaddi-tion operators add and sub-tract time intervals from a date datediff() returns the number of days between two

dates (Listing 15.49).

■ Alternatives to date_format() are the extraction functions extract() , second() , day() , month() , and so on.

449

Listing 15.49 Working with dates in MySQL.

SELECT

date_format(current_timestamp,'%s')

AS sec_pt,

date_format(current_timestamp,'%i')

AS min_pt,

date_format(current_timestamp,'%k')

AS hr_pt,

date_format(current_timestamp,'%d')

AS day_pt,

date_format(current_timestamp,'%m')

AS mon_pt,

date_format(current_timestamp,'%Y')

AS yr_pt;

SELECT

pubdate + INTERVAL 2 DAY AS p2d,

pubdate - INTERVAL 2 DAY AS m2d,

pubdate + INTERVAL 2 MONTH AS p2m,

pubdate - INTERVAL 2 MONTH AS m2m,

pubdate + INTERVAL 2 YEAR AS p2y,

pubdate - INTERVAL 2 YEAR AS m2y

FROM titles

SELECT datediff(date2,date1) AS days

FROM

FROM titles

SELECT

(year(date2)*12 + month(date2))

-(year(date1)*12 + month(date1))

AS months

FROM

(SELECT

FROM titles) t1;

Listing

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