sql 语句的问题，能解释一下AS、LEFT OUTER JOIN的用法吗？

zhuyj

SELECT  mlbrno,mlsbno,mlcuno,
mlname,mlblde,mlibno,dttrdt,mllnac,dttrtm,dtctfg,
dttacd,dtcdfg,dttram,dttrtl,dtacbl FROM
(SELECT mlbrno,mlsbno,mlcuno,mlname,mlblde,
mlibno,mllnac FROM lnmla
WHERE (mlstcd = 'A')
OR (mlopdt >;=:tpopdt AND mlstcd='C')
OR (mlcldt >;=:tpopdt AND mlstcd='C')
) AS M
LEFT OUTER JOIN
(SELECT dtacno,dttrdt,dttrtm,dtctfg,
dttacd,dtcdfg,dttram,dttrtl,dtacbl
FROM lndta WHERE dttrdt >;=: tpopdt
AND dttrdt <=: tptrdt AND dtstcd='A'
) AS D
ON mllnac=dtacno